I think I partly agree with OP.
The thing I don't really agree is putting Smash Ultimate on the same boat as MK8DX since assets reusing is something that every game does. Tears of the Kingdom, GoW Ragnarok and Horizon Forbidden West I suppose, etc. I think single player games get away with it easier due to new stories.
Also, a lot of the characters got new models, not only that, many got redesigns. All stages were reworked too.
But, with both games, the thing I can see happening is them not being early life Switch 2 titles. And as the top 1 and 3 selling games on the system, and with Drake probably being BC and hopefully games getting patched to support 4K, I hope/expect that after the patch they'll have more content. Not another full fighters pass or another 48 courses, of course, but just some appetizers thrown around 2024 and early 2025.
Mario Kart Next I can see being the holiday 2025 game, but I don't think Smash can come that soon. For it to be, we'd have to assume it started development right after DLC for ultimate wrapped up and that's unrealistic to say the least. A game that's more in the table than "Ultimate with some cuts, some new characters and stages and different story mode and all assets reused" wouldn't be done that fast. Ultimate could be made in 3 years because Sakurai makes miracles(as in overwork) and it had the single biggest development team ever at Nintendo + reused several assets and scenarios and movesets from Wii U.
I think that whenever Drake comes, we get patches for 4K support from key titles ( TotK, MK8DX and MP4 being highlighted/getting more than just res bumps), a new "half BCP" is announced for MK8DX(new courses til the end of 2024 or spring 2025), a new fighters pass is announced for Ultimate. Mario Kart Next releases holiday 2025. A new Smash, probably 2026 imo.
We don't know the team size of MKT dev team. The only info comes from patents and I think we have say the same for Booster Course Pass.
I don't think we have any reason to assume MKT took away less resources/staff than MK8(which was miraculously done in only less than 2 and a half years).
It's not like it's a Super Mario Run or Fire Emblem Heroes case of the mobile game clearly being way lower budget and requiring way less staff than a main entry. Tour is a main entry. It in fact has more content than any Mario Kart game ever. And it doesn't reuse assets, the remastered tracks are all made from the ground up.
BCP being just the Tour tracks remastered(as in reworked in some areas, with new lighting and higher res/poly), kinda shows how it's pretty much a console game effort.
It had continuous development which is why I don't really think a next Mario Kart started full development in 2019. Almost certainly started pre-production by then tho. Pandemic slowed things down too, and I wouldn't be surprised if during that time, games meant from the start to come in 2024+ weren't the priority of were Nintendo would put EPD staff.
The continued development of the game slowed down really, so probably the team of post launch support was smaller, and so was that of BCP(which would just need to remaster the tracks in MK8 engine), so during that time(and of course by last year/now) full development must've started and progressing well.
I believe the next Mario Kart will heavily rely on graphics and add new mechanics, and also have the biggest package from the start(I mean before DLC) and maybe feature some adventure/story mode.
Don't expect it before holiday 2025. If only because that might end up being the first full next gen game/game meant for next gen from inception by Ninty, not much different than how MK8 was mind blowing in 2014. THAT was what I'd call HD development mastering. The game is stunning to this day.