I really don’t get the “should’ve been on Switch because 3DS is old”-argument either.
If you want to play a new Metroid, why care? It’s a new game, it’s a good game, it’s a game in a franchise you claim to like or are even a fan of. You have the system anyway so why does it matter on which system you buy it? Is it the graphics? Have people actually stooped to the level Nintendo fans usually claim to look down upon?
Why is it, in general actually, that a system’s predecessor is always a complete no-go for playing any game and is immediately thrown in the trash, while at the same time people are going nuts for games and systems that are even older? Why do you play Metroid on NES Classic with a big smile on your face but puke at the sight of Samus Returns on 3DS? Last I checked, Metroid for NES is a lot uglier and has a lot more primitive gameplay mechanics and control schemes than Samus Returns does. This is not to say you shouldn’t play an old game, because it can be fun, but for the same reason don’t you want to play a new game?
Maybe you’re just not a fan at all, because if it was me, Nintendo could release Breath of the Wild 2 on NES (or 3DS) and I’d still get it day one. Hell I’d scour the Earth for a Virtual Boy if need be.