I do get the impression that some (not all) of Miyamoto's philosophies about video game design are outdated, and were even by early 2010s standards. But having said that, the Wii U failed because Nintendo got everything wrong with both the console's overall design (I will unironically argue that from a pure hardware standpoint, the Wii U was the worst console ever produced by a major manufacturer) and the advertising. As influential as Miyamoto was at the company, I have a hard time believing that he had that degree of influence.

Miyamoto's influence is significant, but perhaps overexaggerated by many. There were those at Nintendo that thought the Wii U would sell 100 million.

At Nintendo itself? I remember reading that Kimishima expected the Wii U to flop and the Switch to sell as much as the Wii. That guy really knows what he talks about.

The Wii U had a bunch of problems. But the way Miyamoto was running EAD during this time very much contributed to some of the big ones. Being the head of what was at the time, Nintendo's largest division, that created most of its most important IP, Miyamoto obviously had a lot of say in how the Wii U was designed and presented, as well as what software he and his teams would be making for it, and when they would release.