This isn't really anything new for Nintendo, though. The Gamecube -> Wii -> WiiU were compatible. The Gameboy Color etc. were backward compatible with the Gameboy. The 3DS was backward compatible to the DS.

It's actually quite a gamble to introduce an entirely incompatible system. Now that Sony and XBox are just x86 with an AMD graphics card, it's hard to imagine them not being backward compatible from here on out...