It would be sad, sure, but it probably wouldn't impact the bigger picture all that much if one player dropped out of the industry. It's happened many times before.
Personally, I would miss Zelda, but I've already played enough platformers to last me a lifetime. Mario being gone would make zero difference to me.
One thing that I think would happen is that one or more publisher would try to plug the hole with more Nintendo-like software on the remaining consoles and especially tablets. It's possible that a good amount of Nintendo customers would become iOS/Android gamers.
The big question would be, what happens to Pokemon? If it disappears, that would probably be the biggest blow to gaming. If Pokemon survived post-Nintendo on tablets then I think people would transition much easier.
