I think Mike Mearls missed the point. OGL was not going to evolve or improve D&D. The fragmented market he talked about, with different solutions to different "problems" is actually the logical result. This fragementation was productive. He used an FTP client as an example, but RPGs are more like "an OS," and there are a zillion of them with slightly different characteristics that are aimed at different requirements. FreeBSD is not the same as Linux, or for that matter, BSDi.
In a relatively short period of time, the OGL resulted in Conan, Mutants & Masterminds, and Arcana Unearthed, then rapidly spread over to the Runequest games and other stuff. WotC's own Unearthed Arcana became wildly popular with GMs. 3.5 grew and flourished, and the third party market along with it.
Then WotC decided they were done with it. Mike Mearls is declaring the OGL a "failure" basically because his paycheck comes from people who want it to fail. I'm not saying Mr. Mearls is a bad person or deluded or whatever, just that people's opinions definitely reflect their reality. The OGL and similar ideas are very threatening to traditional corporate mentalities, and that definitely describes Hasbro.
WotC is wondering, "What will this show on our balance sheets in the next four quarters?" Whereas hobbyists want to know, "What's the future of gaming? This year? Ten years from now? For my children?"
Corporations tend to be conservative; the OGL is hard to quantify. Corporations are territorial; the OGL questions some assumptions about value.
WotC is hoping people will want to ride 4e's coattails. That is a reasonable expectation. But what happens when WotC wants to hitch a ride on the OGL? The cat really is out of the bag. The potential was there before for open gaming, but with the biggest player not joining in the game, it was a stiff climb. That barrier is gone. 4e remains inaccessible, but it cannot remain that way forever or it will be abandoned by this new generation of hobbyst/designers Mearls talked about. Who will there be to write 5e?