The hardcore and casual dichotomy doesn't make sense either. There's a much better reason why Wii owners didn't transition to the Wii U and it's incredibly basic. If you consider it true that the Wii was liked in part because of its motion controller, then the lack of a motion controller for the Wii U console should be a big red flag. The Wii U wasn't a better Wii at all, so it's no surprise that people who liked the Wii didn't like the Wii U and it has nothing to do with being hardcore or casual. You mention in your article that the Wii U supported Wii controllers, but that's only partly true. Many games don't support those controllers at all, so the Wiimote is to the Wii U what the GameCube controller was to the Wii. Given that Just Dance 2020 will be released for the Wii, I don't think there's a strong lack of loyalty among those so-called casual Wii gamers. The problem with the Wii U was that it was a very different console in comparison with the Wii.
So much this. I want to add: not only many games didn't support Wiimotes, theres was no package of WiiU with Wiimotes included and a dedicated motion control game came only somewhat later with Wii Sports Club. Which was a HD-port of Wii Sports and was packaged in small chunks and only sold digital. Wii Sports Club at launch and a package of WiiU with Wiimotes or of Sports Club with Wiimotes (and yes, Sports Club as physical game) would've made a major difference. Not Wii-level of success, for that the WiiU was too different. But probably at least double as much hardware sales - which would've also lead to better software sales.