Top 10 best selling Switch games.
1) Mario Kart 8. - Wii U Port.
2) Super Smash Bros. Ultimate. - Wii U Port.
3) The Legend of Zelda: Breath of the Wild. - Wii U Port.
4) Super Mario Odyssey. - Switch Exclusive.
5) Pokemon Sword and Shield. - Switch Exclusive. (Probably a ported unreleased 3DS game going by it's poor visuals.)
6) Animal Crossing: New Horizons. - Switch Exclusive.
7) Pokemon: Let's Go, Pikachu! - Switch Exclusive. Remake.
8) Splatoon 2. - Switch Exclusive.
9) Super Mario Party. - Wii U Port.
10) New Super Mario Bros. U Deluxe. - Wii U Port.

50% of the Switch's best selling titles ARE ports from the WiiU.
Arguably, the WiiU's issue was not game quality or popularity but likely quantity and poor hardware.

https://en.wikipedia.org/wiki/List_of_best-selling_Nintendo_Switch_video_games

Smash Ultimate isn't a port; it takes A LOT of the framework of Smash 4 Wii U as a baseline, such as engine, art style, animations and move lists, but it's as much a port as Splatoon 2 is to the original. There is plenty of new enough in it to justify it's status as a sequel.

I say this as someone who would have been just fine with a port of it.

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.

Starfox Zero was the kind of thing that is better be forgotten. Forcing the dualscreen when it's not needed and making the game worse with messy controls is a big no. Considering how bad was the reception and the sales, I gotta say that Miyamoto regretted even thinking about this

Eh. I consider it an enhanced port due to the re-use of the engine and so forth, allot of effort went into it though, no doubt.

Can it still stand as a standalone title? Sure. But I would argue that allot of the added-extras in the Switch version could have just been WiiU DLC.

I do wish I could see BotW running as originally planned on WiiU. But, that's all.

Switch is more powerful than Wii U so some games just would not run on Wii U that can run on Switch and some of those running on Switch are at the bare minimum. The other thing is Nintendo used its own API for Wii U. Same with Gamecube and Wii. GX. Not widely supported. Switch uses Nvidia NVN which supports OpenGL and Vulkan. Wii U CPU was a beefed-up Gamecube CPU essentially. Take the bad Wii U marketing aside and Wii U still would not work from its hardware and API alone. Wii U was a half step towards Switch. There is no universe where Wii U is a success story. Nintendo often tries things several times. Motion controls with U force on NES. 3D with Virtual Boy. They tried again with Gamecube with Luigi's Mansion but the glasses were never released. Dual Screens. Game and Watch. Mario Maker was meant for the Famicom. Mario Factory. Mii's were planned back then as well. Nintendo sits on ideas or tries them over and over.

The second screen experience goes back to Neo Geo Pocket Color linked with Dreamcast. Plus VMU. It didn't go well. Gamecube tried it with GBA link. It didn't go well. Sony tried it with PSP and PS3. No one cared. MS almost did VMU with Xbox but dropped it when no one looked at the VMU on DC in focus tests. MS Smartglass was a flop. PS Vita to PS4. No one cared enough. Same with Fallout 4. Second screen gaming is an idea that sounds cool on paper but never worked in execution. DS/3DS only worked because they are right there in your hands. The console side doesn't work.

Oddly Switch is another idea that is old but finally worked. Turboexpress in 1990 was a half step. A portable TurboGraf16 but it could not connect to the TV. SEGA Nomad in 1995 was a portable SEGA Genesis, connected to the TV and had an extra controller port for 2 players. It was an expensive and bad battery life. PSP tried it. PSVita. Some tablets but Switch did it better and had the right timing. Think of it like the Simon smartphone in 1992. A touch screen smartphone in 1992. Many more smartphones were attempted esp in the early 2000s. Didn't take off until everything was in place from tech to form to the world was ready in 2007.

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.

Same for me

I was actually waiting for a tabletop RPG to release on the Wii U. As in, the DM gets the Gamepad and puts everything up there, and the party members use Wiimotes or classic controllers on the TV screen or are connected online with a different display than what the DM gets to see.

It's really a shame that the possibilities of asymmetric gameplay never got really explored with the early death of the Wii U. I hope retro enthusiasts will someday make great games for the Wii U by exploring this deeper and deeper.

Odyssey - I'll give you that

SSBU - This wouldn't be on the Wii U. Wii U already got a Smash. At best, it would be more DLC characters. A six-year old game not named Grand Theft Auto V isn't propelling the Wii U to double its current totals and add a couple of million sales on top of it, even with Banjo & Kazooie and Joker thrown in there later.

The Pokemons - They would be on 3DS; let's not argue this either. There has never been a mainline Pokemon game on a console prior to the Switch, and the Let's Gos and SnS are only on Switch because where else is Game Freak going to put them? If the Switch releases in 2019 or 2020, then they are either on the 3DS or Switch. Zero chance of being on Wii U.

Animal Crossing - If the Switch releases in 2019 or 2020, then it's on the Switch. Zero chance of being on Wii U.

BotW 2- Doesn't exist yet. And when it does, it either gets released on the Switch and Wii U at the same time, or not the Wii U at all.

So if the Switch doesn't release until last year or this year, the only title out of all these that Wii U gets is Odyssey, and probably shares BotW 2 with the Switch at best.Going by VGChartz numbers, Wii U is just under 14 million. Let's say other Switch exclusives that Nintendo funded, such as Daemon X Machina, Xenoblade 2 and Astral Chain all release on the Wii U, along with things like Mario + Rabbids and the most compelling version of Starlink, due to having Star Fox. Remember that any game that has a version on Wii U already can't be counted for Switch, like MK8, Mario Tennis and Mario Party.

Do you think SMO, Daemon X Machina, Xenoblade 2, Astral Chain, M+R and any other exclusive Switch game that I'm forgetting (and Nintendo either published, moneyhatted, or allowed it's characters to be in) is enough to double its current sales? I'll also give you Marvel Ultimate Alliance 3, because I think Nintendo moneyhatted that one if I remember correctly. I'll also give you Travis Strikes Again: NMH, just because.

I'd also argue that certain ports such as the Crash/Spyro trilogies, the Disgaeas, Katamari, L.A. Noire, Rocket League, the South Parks, Valkyria Chronicles, The Witcher, Shining Resonance Refrain, Ys VIII and the ones I mentioned in my last post would not exist on Wii U. Most of those games were around well into the Wii U's life and were never even considered for a port, so there is no reason to believe they suddenly would be greenlit later on down the road, when publishers were long convinced that any Wii U port would bomb.

Wii U is just under 14 million. Would the games I mentioned two paragraphs ago be able to double those sales? I highly doubt it.