"Whatever happens next, that emperor is probably not going to be able to continue ruling from where he left off any time soon. Everyone's seen how all his tricks work."
The problem is that we HAVEN'T seen how all his tricks work.
Nintendo can always come up with new power-ups, new enemies, new mechanics, new terrain types, etc etc that aren't in Super Mario Maker. They can also make 'stylized' 2D platformers--that is, like traditional games of the series but with some huge, main twist--like Yoshi's Woolly World and Kirby's Epic Yarn.
The other advantage of an actual Nintendo Mario game is that you KNOW it's a collection of good levels, whereas with a huge collection of publicly-made levels you need to steal with Sturgeon's Law.
And of course, even if the 2D Mario games stop, they're only a small part of the Mario franchise--there's still the 3D platformers, the sports games, the RPGs, the random spin-offs (like Mario Party), and the sub-franchises under other characters (Luigi, Yoshi, Wario, etc).