By most accounts of which I am aware, D&D had more of the market through use of the OGL than they had since the days when they were the first and one of the few RPGs on the market at all. They only gave away that advantage again by turning their back on the business model that got them there and re-instituting the isolationist policies that saw that market slip away in the 90s. Now they are trying a new approach to bringing everyone back into the fold. Whether that will work as well as the OGL business model remains to be seen. The OGL model remains a very strong one without them being on board. Now they will be competing with their own business model. Now they are going all in against a strong hand that they know they do not hold. We will see what happens.
depends on how you look at market share.
Were there more RPG books out there? Sure. But did that put money in WoTC"s pocket or in the pockets of game store owners and 3rd party publishers?
I know there were lots of times I bought 3rd party books INSTEAD of WoTC books. Primarily because I thought they looked more interesting. But for whatever reason I went to the store to buy a new gaming book, browsed around and decided to pay someone other then WoTC. That doesnt help them.
Is it anecdotal? Sure. But bet most people here can think of lots of times they did the exact same thing.