not so. If Activision were to drop support for PS3, then M rated titles that have fairly high sales numbers (Modern Warfare2 comes instantly to mind) would be deeply sliced. Sure the Wii is good to develop for if you like making crappy titles or yet another guitar hero, but it sucks for immersive in-depth 3d action titles.
Activision could indeed drop PS3 and be ok, but I don't think they will. Kotick ain't stupid, he's just got big greedy balls.