...comment on your response to the following scenario:
1. 4E is successful, maintains itself as the top selling and most played RPG, and runs 8-10 years in its current direction before being replaced by a 5E even less like previous editions
Sonds very likely, though I expect a time frame of 6-8 years, rather.
2. Pathfinder tapers off after a successful launch by 3PP standards, and achieves a stable presence on par with True20 or Mutants and Masterminds.
PF will probably fare better than this. On the scale ranging from True20/M&M to 4E it will be pretty close to True20/M&M, though. What this means to Paizo is another matter; it could be a success for them anyway.
3. The 3.5E playing community shrinks over time until its on par with people playing previous editions.
Very likely so. Of course the existence of the internet means that the game as such can be better maintained by the community. If you wanted to stay true to 1e in 1989, you'd to be lucky to know other players in your area who wanted to do so, too. Today you can play over the net with like-minded individuals from all over the world. Another thing is the existence of the OGL, which, especially on a global scale, allows publishers to support core.3.5E gaming.
4. OGL based gaming begins a slow decline, with the big names soldiering on and fewer and fewer new products being released.
If you mean OGL based as in using the 3.5 SRD, your probably right. On the other hand, the OGL has introduced something new. Publishers other than WotC, notably Mongoose, have developed games using the OGL, but their own SRD instead of the WotC one. This re-introduces the original spirit of the OGL, namely that of a joint effort by several publishers/individuals to support a game. I fully expect this way of publishing to grow, not to bring on the game which will win over D&D, but to provide a market for smaller games, which may leave the obscurity behind, which has been the fate of many games of the path.
Ah, isn't it fun to speculate wildly?