Eh, I don't see it. Keep in mind that many of the current games that Switch has today are Wii U ports, and that Nintendo gets away with them practically being new games in the eyes of a lot of gamers because those same games couldn't get them to buy a Wii U (look at Pemalite's post).
Also keep in mind that the vast majority of the third party support on the Switch that aren't pure indy games flat out would not be on the Wii U: No Skyrim, no Dark Souls, no Dooms, No Wolfensteins, no Hellblade, no NBA 2K, no Street Fighter, no Tales of Vesperia, no Ni no Kuni, no Okami HD, no Dragon Quest XI, none of Namco's anime licenses, and so on. Most of those games could have been on Wii U a long time ago, but they never came. Wii U got one 2K, and that was it. And the third party ports that Wii U did get (and would possibly get in this hypothetical example) would be the worst selling versions, because people wouldn't see a point in getting them over other consoles. The Wii U doesn't have the portability factor of the Switch that makes the latter a gamechanger.
It's not a reach to say that exclusive games like Octopath Traveler and the upcoming Shin Megami Tensei V would at best be afterthought ports, but probably would be Sony exclusives as well without some serious moneyhatting from Nintendo.
Would the Wii U even be able to get to 20 million at this point if the Switch didn't exist yet? I doubts it.
With Mario Odysee, SSBU, the Pokemons, Animal Crossing and BotW 2, I am sure it would have broken 20 millions.
And these mentioned games would have looked nearly identical on the WiiU.
Current PB on Secret of Mana remake : 2h27 (2nd)
Strongest worldwide achievement on TGM : 1st European S13 rank
Fastest TGM3 MASTER in Europe : rank Master V in 5min10
Western record on TGM3 EASY : 1484
Current PB on Power Ranger (Game Gear) : 10min06 (World Record)
Non-geek activity : ThermalHungary