In that case, yes, I would say that is their core customer base. Or at least, that it was; I think they've developed their own fan community to the point that even an official WotC re-release of 3.X D&D wouldn't put them out of business.
Not without tinkering with 3.x, IMO.
I mean, that 3.x release would have to be 10 times more charming than that Arnold on Green Acres to sway Pathfinder players into playing 3.5 again.
All of the Pathfinder/4e talk aside, let's not forget that there is a sizeable number of 3.5 players who never stopped playing that.
As for:
I'd probably shop around too if the game I was playing was going to be canceled and there were similar games out there that were going to be well supported into the future.
Given how many 4e books are released, why on Earth would there being or not being future books matter?
It's the exact same 3.5 argument, only for 4e, and it's just as much of a non sequitor.