It seems to me there are two main lines of thinking going on here both of which are right from their vantage point.
Nintendo fans who (rightly) say once the games come out, of course we'll buy. And I'm sure no one doubts that when Mario, Zelda, Smash Bros, Kart come out Nintendo fans will buy the system and it's sales will rise above current levels.
The other line of thinking is that ONLY Nintendo fans are going to buy WiiU and that makes it - by default - a sales failure along the lines of GameCube/N64 (20m-30m). While it's possible some core exclusives or casual craze will get people to buy, both of those seem very unlikely to happen and either would have to be strong enough to overcome the WiiU's inherient weaknesses - low power / non-casual friendly interface. It's not that this group is saying WiiU will always sell as poorly as it is right now, but that it will never catch on in a big way and compared to Wii, PS3 and 360 will never appear as 'successful'.
WiiU will almost certainly be a profit generator for Nintendo but still leave Nintendo in a very bad place with both the 3DS and WiiU most likely to continue to underperform outside of Japan, it's brand and relevance (and profits) are waning.







