Would a regular console be a safety net for Nintendo, though? Their sales have been continually decreasing. If the Wii were to take the fallback option it'd doom itself since it would just be a weaker copy of the other consoles. The GC had no defining feature, it was just a PS2 with less games or an XBox with no online. The Wii needs a defining feature, i.e. something that can be summed up in one sentence as the reason to pick a Wii over the other consoles, something no other console does better. The big uncertainity and low acceptance with the XC and PS3 right now also bases itself in a lack of defining features. What makes the XC unique? Some toys in the Live featureset? The PS3? Oh, right, Riiiiidge Racer and real-time weapon change. That's possibly the biggest reason people are in awe over the Wii right now, the Wii has a defining feature, a feature you can name and people will say "yes, that's the Wii".
And really, I don't think it'll fail. Even if it's not used for new gameplay mechanisms it reduces the abstraction from the interface allowing the people who find current controllers too complex to play a game. Someone can forget which button turns left or does a barrel roll but they would never ask "which way do I have to move this to move left?". Each gesture is a distinct action whereas pushing a button only differs in where that button is and that location rarely bears any relation to the purpose of the button. Why do you think games that only use the mouse and one button are so popular with the casuals? Only the cursor movement needs to be remembered, the rest is interacting with a GUI.