Nintendo really doesn't make "hardcore" game machines in the sense that every Nintendo game machine is aimed first and foremost at the family demographic. The N64 is probably the most "hardcore" game machine they've ever made, and even by the end of that generation, you could get the N64 is more candy-colored models.
But saying "well just make a new Wiimote like craze" is kind of a non-sequitor, it's like saying to make a successful basketball team you just need Michael Jordan and another good player. Or like saying to be rich you just need an idea like Facebook. Well ... yeah, no duh. Fads like motion gaming come around once every 10-15 years IMO.
Saying you need that and actually doing it is two different things.
Yes a Fusion device will probably sell less than a Wii U + 3DS ... but even if it sells 80-85 million (between all variants), it will be a strong success and make Nintendo a boat load of money as long as they're not dumb enough to anchor it with some expensive gimmick.