It's unlikely that Apple's support for USB had any significant impact on the adoption of the standard.
There are two other events that, in combination, "jump started" USB adoption.
1. Windows 98 release provides comprehensive USB support
Windows 98 was released in June 1998. Prior to Windows 98, USB support in Windows was flaky at best. Support was hastily added to Windows 95 through a patch which was pretty universally panned at the time, with some vendors even disabling the ports if the machine shipped with Windows 95:
We spent hours trying to get the Panasonic notebook's USB port to work, then found out the port had been disabled. Panasonic believes that USB with Windows 95 OSR2.1 is so unreliable that users are better off without it.
If all goes well, the next version of Windows may help. "We acknowledge that support [for USB in Windows 95] is very limited," acknowledges Stacey Breyfogle, a Microsoft product manager.
Vendors must write complex drivers to make their USB peripherals work under Windows 95 OSR2.1, but Microsoft will build more of the USB technology into Windows 98.
Windows 98 added this extra functionality, fixed many bugs and made it easier for vendors to write robust drivers for their devices:
The Windows 98 and Windows Millennium Edition (Windows Me) operating systems contain second-generation Universal Serial Bus (USB) support from Microsoft. A fundamental assumption in the PC industry is that second-generation software is always more stable and has fewer bugs than the first-generation version. This is partly because the availability of first-generation software encourages IHVs to produce a greater number and variety of devices; therefore, as the second-generation software is developed, it can be tested against many more devices.
This is particularly true for USB device support; only a handful of devices were available for testing the USB support first released to OEMs in October 1996 (in the release known as OEM Service Release [OSR] 2.1). In contrast, the USB support in Windows 98/Windows Me has been tested against over 100 production, pre-production, and engineering prototype USB devices. In particular, more interoperability testing has been done, with multiple USB devices attached to the bus at the same time. In addition to stability and fewer bugs--and therefore a better user experience with USB--USB support in Windows 98/Windows Me has significantly more functionality than the first-generation release.
2. USB 1.1 Specification release
The USB 1.1 Specification was released in August 1998 (though peripheral manufacturers would have had access to draft revisions much earlier than that in order to have 1.1 devices ready for market). The USB 1.1 spec had "Updates to all chapters to fix problems identified".
Texas Instruments' assessment of the adoption of USB identifies the updated standard as a key driver for vendors:
The release of the USB 1.1 specification combined with the native operating system support offered by Microsoft enabled the rapid adoption of USB hosts in the PC. It also drove the conversion of many peripheral devices from legacy interfaces such as serial (RS- 232), PS-2 (mice and keyboards), and parallel ports (Centronix and IEEE-1284 for printers) to this common interface standard.
ComputerWorld released an article in December 1997 which describes in more detail how peripheral manufacturers were waiting for Windows 98 USB driver support before fully backing the standard:
Universal Bus awaits Windows 98 drivers
...Most notebook PCs have come equipped with a USB port since mid 1997, but most vendors aren't yet making the printers, scanners, cameras, mice or monitors that comply with the USB standard.
But at Comdex/Fall '97, held recently in Las Vegas, a handful of vendors displayed USB-compliant devices. They included Eastman Kodak in Rochester, N.Y.; Intel Corp. in Santa Clara, Calif.; Logitech Corp. in Fremont, Calif.; and Connectix Corp in San Mateo, Calif. And 3Com Corp. announced that its new 56K bit/sec voice/fax modem will have USB support.
However, the "one-size-fits-all" port isn't expected to be easy to use until end users upgrade to Microsoft Corp.'s Windows 98, which has the necessary drivers to support USB devices. Windows 95 doesn't fully support USB.
"We're getting close to this being a real product and a real standard with support", said Nathan Nuttall, an analyst at Sherwood Research in Wellesley, Mass. But he said corporate adoption of USB will take two to three years.
"If you've got a system [in place], you're not going to throw away all your monitors and printers", he said. "For now, you're looking at a lot of serial connections."
Published just after the release of Windows 98, this CNET article does a good job of summarising the numerous benefits the new OS would bring to peripheral makers:
Peripherals to surge with Win 98
Peripheral vendors--companies that specialize in modems, digital cameras, add-in cards, and the like--will likely experience an upswing in business following Microsoft's Windows 98 rollout, since the new operating system will bring built-in support for a number of emerging hardware technologies.
By far the most practical addition to Windows 98 is built-in support for the universal serial bus (USB) connector....
"Up until now, there has been no reason to implement [USB] on low-end products because it adds to cost," Bursley noted. "That said, we expect within the next year some inkjet printers will offer USB connections. Many already offer USB printers in Japan," she added.
But peripherals manufacturers are working on USB devices as PCs, toward the day when Windows 98 and USB connectors become more common. Eventually, economies of scale will help persuade companies to come out with more USB products.
"There will be a pretty impressive showing of peripherals this summer," said Rob Bennett, group product manager for Windows 98. "There are 250 devices due to be launched around Windows 98 and 100 in development [to be] released in the next year," he said.
John W. Koon's book USB: Peripheral Design summarised the landscape for USB peripheral manufacturers in 1998:
According to Dataquest and Intel's projectsion (USB conference, July 1996), USB PC shipments were estimated at 20 million units in 1997 and 100 million units in 1999. In addition, the Bishop Report stated that the USB connector market would hit $400 million in 1999. The estimated ratio of peripheral use per host PC is four to one.
Mid 1998 saw an explosion of USB devices onto the market. According to the USB implementers forum chair at the time, Stephen Walley:
Three main factors are credited for the large volume of development activity, according to Whalley. These include: USB becoming mainstream on all consumer desktop and most notebook PCs, the wide availability of building blocks for developing products (such as silicon and development tools), and the upcoming availability of Windows '98.
What About Apple?
Apple's release of the iMac spurred the recovery of the company, but it's difficult to argue that a single computer model (accounting for less than 5% of private desktop computer sales at its peak) influenced the adoption of USB in any great way.
In 1997, Steve Jobs had recently been re-hired as Apple CEO and was trying to put the company back on the path to success. He slashed their product line and penned a deal with Microsoft to get Office on the Mac for at least 5 years, and announced the iMac which did away with all Apple proprietary ports and offered only USB.
USB at the time didn't support high speeds - the aim was for Apple to be interoperable with the world of PC peripherals that was largely incompatible at the time. Apple users had to buy Mac-specific scanners, printers and webcams, and Apple moving to USB would make it easy for peripheral manufacturers to make their products cross compatible, which made the Mac a more attractive platform and made it easier for PC users to switch and keep all of their PC peripherals.
The earliest public hint at Apple support for USB was in October 1997:
Jobs' keynote, meanwhile, offered many promises and few details for the bedraggled company's recovery.
... He also said that in 1998 the company will support FireWire, Apple's high-speed technology for linking Macintoshes with devices such as printers and cameras, and Universal Serial Bus, which links peripherals like scanners and monitors.
However, prior to any indication that Apple would support USB, the following had been ocurring:
- Jan 1996 - USB Specification 1.0 released, authored by Compaq, Intel, Microsoft and NEC, see also the full USB 1.1 Specification (PDF)
- Nov 1996 - PCWorld deems USB a "radical rethink" and "grand idea", and eagerly anticipates devices being made available
- Jan 1997 - Microsoft, in addition to previously helping to develop the USB standard, releases an updated version of Windows 95 enabling the use of USB devices
- Feb 1997 - Kodak demonstrate the first Digital Camera with USB connection for power and data
- Feb 1997 - Xirlink demostrated a USB Webcam which went on sale in Sept 1997
- June 1997 - Philips demonstrated a USB hub enabled monitor, USB infrared port (for already existing wireless mice and keyboards) and USB speakers
- June 1997 - Thrustmaster demonstrated a USB Joystick
- June 1997 - Sony began producing laptops, these had both USB and Firewire ports as standard
- July 1997 - HP announced future computers would be USB enabled, and showed off PCs with USB keyboards which included multimedia buttons, as well as USB printers and scanners
- July 1997 - Canon had functional USB Printers available for testing
- July 1997 - Dell, Gateway, NEC, and Panasonic were selling consumer PCs with USB ports
- Oct 1997 - Connectix (now Logitech) QuickClip USB Webcam available for purchase for $99
- Nov 1997 - Connectix (now Logitech) QuickCam VC USB Webcam available for purchase. Over 1 million Connectix units sold before the release of Win98. This camera didn't work with the iMac until September 1998
- 1997 (Month unknown) - USB-IF membership increased to more than 400. Over 500 products were in development worldwide. First third party USB developers' conference held.
Essentially, all of the major technology players had decided to support USB as a standard and peripheral makers were switching over. Prior to the release of the first Apple with USB support, USB devices and computers with USB ports (for example: the Dell GXa, one of the best selling business PCs of the era, IBM's PC 300 Series) were both widespread.
There were well over 50 USB peripherals on the market prior to the launch of the iMac in 1998, and hundreds more came onto the market throughout that year
Another reason that Apple likely had little impact on adoption of USB was their tiny market share at the time. In November 1998, over 84% of retail sales were PCs just from the top 4 brands at the time, with Apple coming in at under 5%. This also doesn't take into account business PC sales, which were at the time even more heavily lopsided in the PCs favour.