The failure of companies to follow a sound plan does not prove the plan would not be sound. Pathfinder is in its 4th year and still going pretty strong and there is a solid core of 3pp who are making a living off of supporting it. (and by "a living" I mean there are more than 1 where being a 3pp for Pathfinder is their day job.)
I have seen a few providers of 3rd party content, but I’m not sure which ones provide such content as a full-time job. I’d expect that would mean more than an AP every year or two.
I would also suggest that printing a 2nd edition rulebook is not analogous to reinventing your game. Call of Cthulhu has done a pretty good job of sticking with a core set of rules (a ruleset which is flexible enough to support other systems) and of producing support material for the core system, while at the same time branching out. Of the companies following the current WotC model (Games Workshop follows a similar path) I notice growing dissatisfaction and eventual migration to other steadier systems.
The extent of rule changes is a good point. I remember reading 3e and thinking it was a good game, but not the same game it purported to the a 3rd edition of. 4e and 5e deviate as much, if not more, from their own predecessors. But how much change makes it “a new edition” is a tenuous line to draw. There are still BECMI and 1e supporters out there, but I would suggest 1e/2e were the same game, different editions.
As for the OGL, it was adopted by another company (more than one actually) and they are the current leader in roleplaying games. In point of fact, the OGL was even adopted by other companies to support other rule's systems than d20.
Can you explain further what you mean here? Citations would help
I suspect the big one is Pathfinder, but I can’t count Pathfinder, or any company whose work is based on the OGL, as they did not get a choice. That said, Pathfinder has made a lot of its additional rules open source as well. I’m unclear as to where they draw the line in that regard. It is not lost on me, however, that had there been no OGL, there would be no Pathfinder to outsell 4e.