As I said, I think it will take time, as in 3-4 years. I don't see 3.5E recruiting enough new players to replace departing ones to maintain its current popularity.
I don't think it will maintain its current popularity either. However, I think it will occupy a 2nd tier- above games like Pathfinder, True20, M&M, etc.- for quite a while.
And if you take 3.X
as a whole, I think it may rival 4Ed for a while, and only a major non-D&D game like GURPS, HERO or WoD would challenge 3.X games for 2nd place in the market.
IMHO, though, the real wildcard will be whether 4Ed itself can continue its popularity. I've brought up the specter New Coke scenario a few times- namely that a new product hits the market replacing a still-popular product and initially outsells it...but then falters. As it does so, the product it was slated to replace experiences a resurgence.
Its too early to know yet whether 4Ed is converting enough old gamers
and attracting enough new gamers to retain the crown it currently wears. I expect it to- the New Coke scenario is exceedingly rare- but its still quite possible. The main reason I can see that it wouldn't is that, unlike in the New Coke case, the market for the original product is a lot more balkanized.
New Coke lost out to Classic Coke and (eventually) to Pepsi. To be dethroned, 4Ed would have to lose out to 3.5, 3Ed, True20, Pathfinder, M&M/W&W, AU/AE, Midnight, etc.