Well probably the people that realized N64 didn't even hit 40 million while PS1 surpassed 100 million
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Anyway, the Wii U isn't awful like the Wii was but there is a lot holding it back. First the same problem Nintendo always have which is not being able to shake the kiddy image hurts sales in this shooter heavy gaming culture of today.
The second problem is something they will never overcome either and that is the lack of third party support. They did a lot of damage to third party relations when they got arrogant and thought everybody had to bow to them in the industry. They tried to force ridiculous policies on third parties and then Sony came along and everybody they pissed off told them where they can stick their policies
Companies like EA have never gone back short of a few games here and there. Even with the success of the Wii fad they couldn't pull in companies like EA for more than a few games. On top of that they keep releasing weak cheap systems so if a third party would entertain the thought of supporting them they get turned away from the fact that they can't simply port their current gen games.
So when it comes down to it Nintendo have to depend on themselves and there are only so many games they can release. So you are left with big gaps between the games worth buying. Which is sadly an ever shrinking list as Nintendo have been dropping the ball a lot lately. They have also pretty much tossed in the towel on the Wii U and are moving on to NX.
Then we are in an online gaming era which hurts Nintendo. Very few games even have online play on the Wii U while the structure is archaic. And some things are just downright absurd like downloads being linked to consoles and not online accounts. They also lack the bells and whistles that a lot of people have come to expect like achievement/trophy systems.
That being said all this stuff as well as other problems they face with the modern gaming community I didn't bother to post are lost on most kids. So no need to fear buying one for your kids to enjoy some Mario Kart, Smash etc.