Some experts clearly believe that too many new drugs don't bring significantly advances to therapy
In an article in the BMJ in 2012, two experts reviewed the evidence about a perceived crisis in innovation in the pharmaceutical industry. They concluded that there was no crisis and the long term rate of new drug discoveries and introductions had been stable and was not in decline.
But, after reviewing approvals over a significant time period, they remarked (my emphasis):
More relevant than the absolute number of new drugs brought to the market is the number that represent a therapeutic advance. Although the pharmaceutical industry and its analysts measure innovation in terms of new molecular entities as a stand-in for therapeutically superior new medicines, most have provided only minor clinical advantages over existing treatments.
The preponderance of drugs without significant therapeutic gains dates all the way back to the “golden age” of innovation. Out of 218 drugs approved by the FDA from 1978 to 1989, only 34 (15.6%) were judged as important therapeutic gains. Covering a roughly similar time period (1974-94), the industry’s Barral report on all internationally marketed new drugs concluded that only 11% were therapeutically and pharmacologically innovative. Since the mid-1990s, independent reviews have also concluded that about 85-90% of all new drugs provide few or no clinical advantages for patients...
...This is the real innovation crisis: pharmaceutical research and development turns out mostly minor variations on existing drugs, and most new drugs are not superior on clinical measures.
Their paper contains many further references to the primary sources which I won't repeat here.
Their analysis for drug launches since the mid 1990s is consistent with the idea that 90% of drugs don't significantly improve on previous drugs within an existing category (e.g. statins or NOACs). This is at least approximately the claim in the original question.
NB Drugs are complicated things with no two drugs having exactly the same profile of therapeutic benefit and side effects. This creates a large grey area where better depends on how much weight you attach to each benefit and side effect. In many categories this creates a lot of scope for every drug firm's marketing department to claim that their drug is better. Independent experts prefer to count major indisputable therapeutic gains and ignore the minor squabbling about trivial differences (so drugs that treated ulcers were a major advance over surgery, but minor variations in the drugs from different firms were not significant gains).