The MCA bus didn't hurt PS/2 sales that much. Of course, you wouldn't be able to get cheap expansion boards for them because MCA, but they had VGA, mouse, floppy, hard-disk, printer and serial ports, which is what the bulk of expansion boards were used for. What really killed them was that the ISA-based PCs were kind of better, and cost less.

OTOH, I think that the Sound Blaster and the VGA card are the biggest factors in the Amiga's demise. From a user's standpoint (a user who doesn't care about planar graphics, a blitter, and so on), the PC had better graphics and sound, for less, and with all their software still available. Being a peripheral, the graphics on the PC were much easier to change and evolve, and quickly surpassed the Amigas, that had to preserve compatibility with what was, at that point, hardware getting less competitive.