6 years is not a "short life cycle" for any system. At worst it's above average.
3-4 years is where one can cry about a short life cycle.
The Wii U or Wii 2 or whatever they wanted to designate it as would've failed in 2013 or 2014 or whatever. The brand was stale and every system had the same mini-game motion games, if anything Sony's Move controller was by far the best motion controller any of the three released.
If there was any market juice there, they would've just copied whatever it was, things were never going back to the way they were pre-2010 when Wii was the only real motion gaming platform and you had no choice but to buy it to get that type of experience.
It's not a blue ocean when every other console manufacturer is doing the same thing. It's not a blue ocean if everyone and their grandma has touch screen games on their smartphone sitting in their pocket at all times.
I don't think delaying the 3DS and Wii U for a year would've changed anything and you can't redesign a hardware concept in a year's time either.
Nothing you said holds any value when Just Dance 2020 was still released on the Wii. Even with Nintendo's negligence, the Wii still comfortably outlived its motion control competition. The continued yearly release of the Just Dance series proves two thing about the Wii: The console and its concept had much more juice left in it that Nintendo wanted to grant it, and the so-called casual audience wasn't fickle, but reliable.
If it now needs to be explained to you why the Wii U was nothing like the Wii, then it would be just another case of your ineptitude to recognize the obvious. Normal are able to tell the difference between a motion controller and a non-motion controller, and they are also able to tell that the vast majority of games for any given console will be designed for its standard controller.