Nintendo views NSMB U as a "new game" because not many people owned a Wii U, they also view Mario 3D World as it's own in-between series (sorta 2D, sorta 3D) and again since it was on Wii U they kinda view it as a new game as most people have not played it. So I think they view those two games as having a while to sell.
Just like Mario Kart 8 on Switch, it's basically a new installment in the series as far they see it because not many people owned a Wii U so there is no rush for a Mario Kart 9 which is probably being held off for a Switch 2 launch. As Miyamoto says too, they also basically view Super Mario Run as a new 2D Mario game.
It does sound like there is another 3D Mario coming probably around the time of the Mario (late 2022, early 2023) movie release, that's a lot of Mario platforming for one system.
It's also probable they are working on a new 2D Donkey Kong game, I think a Donkey Kong movie is already being planned if the Mario movie does well, Donkey Kong getting it's own attraction at Universal before things like Zelda or Pokemon says a lot. Likely they would eventually like a movie to go with that, and if that's the case, there almost definitely has to be a new Donkey Kong game. There are lots of Mario platformers on Switch, but only the one DKC: TF port.







