Even on switch that demographic is gone, they tried with 1-2 switch, and they tried with labo, the latter now bombed completely (even though it's technically a bit different). Without a doubt the rise of mobile clearly hurt the Wii U.
It's not. 1-2 Switch did solid numbers (will likely do 3-5 million in the end), and Super Mario Party is a huge hit, along with Pokemon Let's Go! attracting a good portion of Pokemon Go players. Labo's a weird case, because it's more of a niche product than either of those, but I also wouldn't say it outright bombed either since all Kits combined are over One million in sales at the moment. We'll have to see what it's fate is When Nintendo gives its post Holiday sales reports.
I'd say the problem is more that the Wii U didn't have anything that made it clear why they should care about it over a phone. The dual screen gamepad gimmick was confusing and un-intuitive, and doesn't have the same appeal as passing a Joy-Con to a friend for Multiplayer. Wii U failed because it was badly designed, not because of mobile.