Nintendo would have to actually go bankrupt before they dropped out of the console market - it's all they do. It's not like they're going to to say "Yeah the console market isn't so hot right now, let's fall back to our other areas of expertise." Unless you count handheld as separate, in which case they could just become handheld only i suppose...
If anyone is going to drop out, it will be one of the other two, who actually have stuff to fall back on.
But i don't see there's any reason why there can't be three companies in the next-gen market. There were three in the last one, and they all did fine.