Launching first, a cheaper price, a stronger Nintendo/Wii brand and a weaker Playstation brand should ensure it does better than the gamecube.
Looking back historically, 50% of the console's predecessor seems to be the biggest drop to expect gen over gen, even when the manufacturer does everything wrong. So assuming Wii U is all bad, it probably would still hit about 50 million unless the market completely tanks or it truly is an unprecedented disaster. Based on the very safe approach Nintendo has taken so far, I doubt they've done enough to truly push away customers.








