Sales would surge across the board at first. Everyone would buy Nintendo games on other systems for the novelty of it. I think after that, however, we'd begin to see certain franchises lose traction.
The thing about Nintendo games doing such massive numbers is that it's partially due to existing in a mostly closed system. Many Nintendo fans stick with Nintendo games out of a sense of loyalty and belonging. Being a Nintendo fan is an identifier for them. Without the hardware, I think that sense of identity would evaporate somewhat and they'd start to buy games based on more than it having a Nintendo badge.
I believe second tier franchise sales would sink downward. Mario platformers would probably hold steady. Games like Mario Kart and Smash Brothers would continue to be huge. The boon would be for Zelda (unless it's like Skyward Sword) and especially Metroid. I think those are franchises that would do very well on other platforms.
It would be interesting to see how some of these "I only play Nintendo games" people would react.
