I agree MK would be a great launch title, but there's a world of difference between single-player Metroid and Mario. If the latter is a launch title, I think the audience for the former is still going to be excited for it touching down, because of the diehard, infrequent, and long-awaited nature.
But if you only had the former, you'd be failing to cater to most of the potential market given the limited waves Metroid makes.
Also one's a kinda edgy, fairly dark (by Nintendo standards) metroidvania/adventure/FPS and the other is, for the most part, a goofy, light-hearted, very child-friendly platformer. I don't think they step on each other's toes much.
But those are exactly the optimal kind of games that sell the system. Which Nintendo is going to want to do.
I think you're underestimating how many people are gonna be down to buy a new Switch after the better part of eight years, especially once they see the games for it. While Nintendo home consoles usually end in a whimper, and the Switch is still going strong, a system doesn't need to be running on fumes for people to be ready for the next thing.
I mean, assuming Nintendo doesn't shit the bed marketing/rollout-wise.
But the priority being the Switch 2 means their goal with software is to push the system and push people towards the system. If everything is a cross-release, then their priority is to have every title sell as much as it can as soon as it can. Which would hinder the former goal, considering it would dilute the draw to the priority. That's counterproductive to releasing a new system.
They're not going to forget about the Switch's install base next year, or the year after that (or maybe even the year after that) but they're also not going to give it near as much high-profile support once the successor arrives.