@ all 3 of you, I'll answer this all at once.
1) I'm having trouble figuring out how many AP monsters so far would be reserved for a Golarion-exclusive setting, when every one of them can be adapted to any setting the DM can think of based on the native environment alone.
2) If they are revising monsters, then I don't see why reprinting them on a compendium would be problematic.
3) If the later Bestiaries were more about new monsters than reprinted ones, then I also don't see why having a 5th Bestiary with on AP monster, only new ones would be a problem either. Budget? Huh... yeah... I don't see a big difference between having to get 90% of the artworks compared to 100%. Again, B4 had like, what, 10 monsters from AP... on 300 or so?
4) I have absolute faith in Paizo to come up with even more new creations... or else we would have never gotten monsters in APs in the first place.
5) How many monsters have been suggested in the "Bestiary 5 wish list" topic again? Believe me, they HARDLY will ever run out of ideas.