Time will tell. I am thinking that Prime 4 will be the flagship launch title. Given the amount of hype and the ambiguous teasers. People want Prime 4 to drop like a bomb and that's the way to do it. And with full backwards compatibility it would be a waste from the business perspective to short sell the new product by making it available on the old one.
And you cant compare all the WiiU-->Switch1 games because there isn't backwards comparability.
Metroid has gained a lot of steam, but it is still no Mario or Zelda in terms of being able to sell a system. Even still, the only waste from a business perspective would be alienating 140m current Switch users just so you can push it on a new system exclusively, especially when the trailer from 6 months ago is marketing it as a Switch 1 game. Regardless of the system being backwards compatible it will still take years and years to get into nearly as many homes as the Switch 1. Nintendo wouldn't really care much if someone buys a current-gen version of Prime 4 or the next-gen because ultimately they still get the sale and can rely on more flagship franchises like 3D Mario, Zelda, Smash and Mario Kart to force everyone's hand long-term.
Prime 4 will likely be part of their launch lineup but there's just no logical reason for it to not be cross-gen at this point. If they really wanted it to drop like a bomb as a Switch 2 exclusive they'd have just waited until this year to finally show off gameplay. Cause even if they do make it a Switch 2 exclusive, they're now gonna get flak for the bait-and-switch of last year's reveal. I'd expect 60/120fps, a higher resolution or just generally improved visuals. It'll function more like Horizon 2 or God of War Ragnarok on PS4/PS5 or Breath of the Wild WiiU/Switch more than anything.
Just really need to stress there is no evidence for Switch 2 exclusivity but there are in fact outright statements that it is coming to Switch 1. Obviously that could change, but there is no logic behind claiming otherwise right now.