The release date for Smash is the single deciding factor to whether or not the Wii U's sales can be salvaged. Think about it. After Smash Bros, what else is there that will be a system-seller for the console? I know Zelda is on the horizon, but Zelda doesn't push anywhere near as many units as Mario Kart and Smash. Those two franchises are it. If MK8 and Smash underperform, then the Wii U will too.
This is why it's so crucial that Smash come out at a good time. And November 21 is the ideal time. With Brawl, Nintendo could afford a delay, because the Wii was flying off the shelves. But I can't stress this enough: Nintendo cannot afford to delay this game past Christmas. The fate of their hardware depends on it.