You don't know that for sure. Remember when the next FF was 'sure' to be on N64? They even made a tech demo for it. Hey, maybe Nintendo was too arrogant, and that led to its relative downfall.
Sony isn't being arrogant at all right now.
All you're "guaranteed" right now is FFXIII and its spin-off. Wii is already guaranteed a DQ and FFCC title. Yes, spin-offs, but you don't get DQ spin-offs being developed by the main DQ team everyday. It can go both ways. Heck, FFXIII might not even end up exclusive. Maybe the 360 will get some love too.
Case in point, PSP vs DS. Squenix' support for the DS is crapping all over their PSP support. They go where the money is. There is no free ride this gen.