The pie analogy doesn't quite work, because it leaves out the issue of player acquisition - something traditionally done by the leading RPG in the industry. D&D is the only mainstream brand the industry has of its own (no matter what the difference in sales between the leading games are), and WotC is in a position to leverage that to grow the industry in ways that other companies can't, although Paizo has made some inroads into that with its Beginner Box and various branding initiatives like comic books, toys, and so on. So a successful D&D means a bigger RPG industry with more players in it.
Plus, of course, actual competition makes people bring their A-Game. Neither company can coast. This benefits everybody!
It is true that the D&D brand name is still the one that brings new players and payers into RPGing than any other. I have no doubt of that (and don't ask me for numbers). It might bring them in to D&D or PF or any number of games that people describe at the table as D&D or D&D-like, but it is their brand name that does the heavy lifting. Frankly, I doubt that will ever change either.
I also don't think the owners of the D&D brand will ever stop having an RPG in production for any significant period of time because I think that would harm the brand, which in turn would harm the licensing of the brand for all manner of things (electronic games, movies, board games, etc.).
I think it is safe to say that the RPG itself, while crucial to the continuation of the brand, is not the majority of the revenue stream for the brand. That essentially ended when they moved away from the OGL, for whatever the reasons and however you assess the fallout of that move. They coincide, no matter if you want to argue direct causality, indirect causality, or even coincidence.
Does the brand owner need to reverse that situation? I don't think they do nor do I think it matters for the larger portion of their revenue pie, nor still do I think currently wish to do so.
There may come a time when someone looks over the market and thinks that the owner of the D&D brand should also be getting the vast majority of the RPG revenue, as they seemingly were doing under 3.XE. At that point we might see something less coy than a GSL or some other bastardization of the OGL, and actually get a straight up OGL game again. Until that time, I personally don't believe we'll see the D&D brand truly dominating the RPG market again, on its own, with no real competition.