Seems dramatic
They could release a nsmbu port this year and super Mario maker 2 next year or 2 and the latter wouldn't be negatively affected.
At all.
Another Mario Maker seems a bit...anti climatic.
Although that's the company that thought WiiMusic and Animal Crossing would carry their end of 2008...
That's only a problem if Nintendo plans to release a new NSMB in the foreseeble future. If they release NSMBU this holiday, then it implies the next original 2D Mario will either be a new Mario Maker or an alltogether different spin on the 2D formula. They aren't going to release NSMBU this year and NSMB Switch next year.
I actually can't follow your argument regarding NSMBU being different from MK8 either. Are you implying MK8 Deluxe only sold this well because it came out very early in Switch's life and wouldn't do as well if it released this year? If that's the case, how do you explain that MK8DX's sales are consistently high? It's still charting number 2 on the Switch charts, so new releases cleary don't affect it negatively.
MK8D is nearly a launch title, it's why it wasn't impacted all that much by expectations (that and it's Mario Kart).
Since MK DS, we know that MK is on a cycle of its own.
I mean MKWii managed 28M before Nintendo thought it was a good idea to bundle it with the system after all so it stand to reason that despite MK8's strong performance on WiiU, it could do even better with little risk.
It was after a fairly low budget release compared to what the same team did afterward with Arms.
MK8 is one of these rare examples of low efforts/high rewards (while porting the game and making the battle mode is no easy feat, it's easier than making a whole new game).
NSMBU is something else, it's kind of games are far more frontloaded and expectations are entirely different after more than 1year and a half.
Also I wouldn't expect more Mario Maker from Nintendo at all, they did say that they wanted to do something new with 2D Mario after all.