NSMBU on switch is selling fairly more than enough for a port. A new 2D Mario would have explosive sales if it's planned.
But what i disagree on with you is what you mentionned about a new 2D metroid. Metroid on 3DS sold poorly because it was a remake first of all on a dying system of a game that had a very good fanmade remake(AM2R). I'm assuming that if it was released on switch it would have done way better.(the same happened with starfox Zero on wii u). Sometimes Nintendo shoots itself in the foot by taking bad decisions concerning some franchises and then put the blame on bad sales. After that they say that no one wants a new 2D Metroid or a new Starfox etc...(and i still don't understand the f*ck that happened with starfox zero seriously).
In fact, i do believe that a new 2D metroid game will do well if it were planned as a sequel for fusion and with the good advertising it needs. Because we have to admit it that Nintendo had never well invested on metroid games. I hope things go well for Prime 4 and see it outsell the first prime (over 3millions sales).
That said, i see that every exclusivity on switch will outsell its predecessors considering the hardware sales including pokémons if gamefreak decides to think about quality the next time they make a game.