The problem was Iwata was obsessed with the 6 year life-span and assumed it was an unbreakable rule for the market. Nintendo did the same thing with the DS, killing off their most successful system ever. By launching too early, Nintendo killed the Wii and the Wii U couldn't get key software out in time. It also made the system outdated by the time Sony and Microsoft released their systems, which made porting more of a chore. With the poor sales of the Wii U, there was no point in attempting it, so the Wii U struggled further.
In the end it was a horrible decision. Nintendo didn't repeat this with the 3DS of course, though I worry they may try it again with the Switch. The lack of new games is feeling like the late Wii days.
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Known as Smashchu in a former life