So regardless of the publisher's intent, if they release books and players buy them - they expect to be able to use them in a game.
I don't relate to this position. I find it both irrational and highly counter-productive to show up at some other GM's game and tell them what is or isn't allowed into it.
Again, if the people playing the game are treating each other like this, then things like power creep are far less significant problems.
When 3.5 was still expanding, a lot of stuff was allowed in my game. But some wasn't. I didn't shy away from saying so. It may sound self-serving, but my players knew that I put energy into keeping the game fun. When one asked me about Bo9S and I said "no", he both respected my reasoning and enjoyed being in the game I ran enough that there was no conflict. I told him *why* I had that opinion, he respected it and we went on with our game.
And I won't be overly concerned, but you hit the nail on the head. If one of my Pathfinder players pays $40 (even though I bought the core, screen, bestiary and Gamemastery Guide) for a book that I am not interested in, what do I tell him? Then I go and buy the book and so on. I would tend to think that most any company would prefer that route, it is only natural, leading to more sales.
That is a much stronger point. Paizo needs to keep selling product. And APs may continue to be the bread and butter, but there is no indication that they think they can stay strong on that one element. And I would think it was a very poor plan if they did.
So, they will produce more books with the intent of trying to make as many people as possible want to buy them. And the need to do this will continuously be a pressure on the system.
Again, to me it is easy because I won't use problem material. And we need to actually SEE problem material before we start complaining about the sky falling. But, if the pressure leads that way, it will be a concern.