PS3:
Blu-ray
Final Fantasy XIII- (the original FF numbered series, the spinofss are something else)
Gran Turismo
Metal Gear Solid
Ratchet and Clank, or losing the developers
Naughty Dog and whatever they're bringing (new J&D?)
The promise of quality Level 5 and SE rpg's
other games like GTA, Silent Hill have been multiplatform anyway, but were they
completely lost for PS3 i'd consider switching sides.
xbos360:
Can't say as the lineup hasn't been able to seduce me there yet.
Wii:
The "wiimote", but I've become undecided if it'll be unique enough after the PS3 gyro controller.
Still, even if all the franchises were lost on my current console of choice, I'm not sure I'd switch to another overnight. There'd sure be other, new franchises being created inside the same genres unless the platform lost all support and popularity at the same time all those franchises were lost (unlikely scenario). So if Final Fantasies were lost, there might arise a new geme series that could be as good as or better than FF.
It'd mainly be a loss of a name and possibly familiar mascots.
And there's evidence when a game or developer has been "lost" to anoter platform, it can lose more than just the key devs in the process...