Windjammer
Adventurer
(My edit came 5 minutes too late! I kept editing my post, and just saw now that you've responded in the mean time.
)

To put it in more simple terms:
I don't believe Pathfinder and Paizo are big enough names to maintain 3.5E at the level it is at now.
Pathfinder and Paizo can be successful by 3PP and their own standards and still fail to accomplish the above.
While I don't deny Pathfinder is an excellent company, it isn't D&D. I would be surprised if after Pathfinder's launch it captured more than 50% of the 3.5E playing community. I'd actually expect the number to be around 20-25%.
You really think Pathfinder will have that many players? I guess it depends on how big (or rather small) you think the 3.x community is, but still?
1. I don't see 4E lasting up to 10 years. While it plays differently to 3.x, it is still heavily derived from 3.x with the powers system (and all it's issues good and bad) grafted on. There is little new that can be crafted for the system except recreating and re-interpreting what has gone before in its new 4E guise. Simplicity and elegance are good design goals but I'm not sure they will inspire the longevity required to string the game out year after year. As such, the likelihood and even existence of 5E is not 100% certain for me. I would not be surprised to see a change of official hands being required to spur on the next edition of the game.
Call it a high estimate.
...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
It's only my opinion so you can take it or leave it.You know, I don't buy this. AD&D 1E/2E lasted from 1977-2000 with less mechanical variety than what is contained in the 4E PHB.