Sakurai pours his heart and soul on every game he works on and pushes the respective systems he works on to their very limits, Kirby, Kid Icarus and of course, Smash. Not just with Smash 4.
Nothing about the Wii U version took advantage of the system's capabilities. The Wii U gamepad should have been used as more than just another controller-type for playing the game. Touch-screen menu would have made menu navigation easier in times. Being able to customize movesets and controls on the gamepad
while everyone else could stay on the character select screen on the TV would have been amazing and very convenient. Instead of Smash Tour, a sort of boss-controlling mode of one gamepad player versus up to 4 regular fighters would have been a
much more fun party game with greater replay-ability. However, none of these possibilities were explored, and we effectively got a game whose only hardware-designed improvement compared to previous entries was its processing power. The Stage Builder was the only thing that made exclusive use of the gamepad, and even that received mixed reception compared to the builder in Brawl.
The point being, the way the Wii U version was executed, any of Xbox One, PS4 or PC would have been able to handle just as well. Smash of course won't ever be on one of the other systems, but to say it was working the Wii U to its very limits when the game itself didn't take advantage of the hardware it was presented with is a shoddy statement. A strength the Wii U had over 3DS was processing power, obviously, but it was its asymmetric multiplayer potential--something that games like Runbow, Star Fox Zero, Game & Wario and Nintendo Land took the most advantage of--that was ironically taken more advantage of in the 3DS version despite the Wii U being a console
created on that very premise.
The Wii U version was still fantastic in the core regards of the main gameplay and the additional 8-Player Smash, of course. I'm just saying it certainly has a lot of missed opportunities that we might not ever get to take advantage of on the Switch version of Smash, regardless of whether it's a port
or full sequel.
Just a bit of Devil's Advocate to that statement, by the way. I won't deny that Sakurai works hard on the series.