I really can't tell whether there are serious flaws in Wii U or not, but having planned to use the same Wii brand, Ninty killed Wii far too early. Instead of seeing it slowly declining as it was time for its successor, people saw Wii suddenly plummeting and rotting by itself, when 7th gen as a whole was still strong, but the Wii brand was actually dead and buried.
It's not just matter of lack of 1st party support, but also awful marketing, Iwata went out of the blue declaring Wii dead and obsolete so long before launching Wii U that when the latter eventually launched, people didn't care anymore.