I think 50 million is doable. But considering PS3 doing 55-65% of PS2 is regarded as abject failure by those who like to see the worst in things, especially anything Sony. Wii U doing 50% of Wii should be seen in the same light.
However, potential should be taken into account. PS3 had the potential to sell upwards of 120 million, but it ended up being handicapped by various factors. I never thought Wii U had Wii-like sales potential, purely for the reason that millions of people who've never bought a console in their life bought a Wii, and the likelihood of that segment of the Wii market becoming multi-generation console owners was always pretty low.
The people who will buy a Wii U are: hard out Nintendo fans (25 million); the multi / all console crowd (10 million?); some of the Wii's blue ocean crowd (10 million); and 5 million more for good measure. Anything substantially more than that is gravy for the Wii U. So Nintendo's job really is to bring Wii U to per unit profitability ASAP so they can get the vast majority of Wii U's sold at a profit, not relying on people buying at least one game to turn a profit.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
