I agree with this where 3DS ports are concerned but not really for the Wii U. How many top-selling titles for the Wii U didn't have Pro Controller support? Smash did, as did SM3DW and NSMBU, same with all three Zelda releases, Bayonetta, Yoshi's Woolly World, Monster Hunter and Pikmin. The only major titles I can think of that absolutely have to use the GamePad is Super Mario Maker an Kirby.
I know what you mean in an "absolutely have to " sense. (the second screen often did something, so the port would need a little jigging in order to disclose its functionality rather than just being able to load the original code into the VC emulator) So I agree with you that it's doable, and I really hope they do! Some of the games I actually liked having the second screen for though (for example, while you could play both Hyrule Warriors on just the GamePad screen, I did really prefer being able to have the second screen available for stuff like maps and easier weapon change access, etc.
So while I certainly wouldn't say no to seeing those ported in a one screen way if it meant I could play them on the Switch (and would strongly encourage them to try to bring those games, and as many WiiU games as possible), ...but what would ultimately be even more amazing is if they figured out way to even do both screens. In a way, it's theoretically possible, because they have both screens just like a WiiU, but its current limitation is that the console bit has to be docked in order to play on the TV screen.
However, I've already seen that people have connected a female to male TypeC extension to the dock via the actual docking connector bit, so that it can be charged out of the dock and play on the TV, so technically that could work too.
However, naturally this auto cuts out the handheld screen but when you do this. But when you consider the auto screen toggle that kicks in when you dock one is software based, this too could even be fixed by a software update.
I honestly don't think they ever plan to do this, and is mostly just wishful thinking on my part. However, I would love to see this happen, and it would open up a whole tonne of possibilities! So Nintendo if you're listening, there's some money to be made here on WiiU games for the Switch that you may have never realized was possible until now! (just floating the idea out there!)
😉