No Remake?

Well, if they wanted to put a remake on the Switch, Partners in Time is the best option for that (assuming they'd already finalized Superstar Saga being on 3DS). It doesn't rely as heavily on two screens and touch controls so it would be easier to rework for a single screen. There are some sections where the babies go to a different screen but I feel like those can create a divider down/across the middle of the screen so you can see both sections without too much trouble, and as for the battle actions that use both screens they can just zoom out like the Superstar Saga remake did for some Bros. attacks. Bowser's Inside Story would be much trickier to implement on Switch thanks to its heavy touch controls (which Nintendo is keen on not requiring handheld mode for Switch games) and a much longer time spent with two screens in play.

That's just an idea of why they could be releasing BIS first, if they are making PIT for the Switch. Personally I think a better plan would've been making Partners in Time + Bowser's Inside Story as one game, with the two modes being the two games, instead of doing another Minion Quest thing, but we'll see how that goes (also for a standalone PIT remake I'd like the side mode to focus on Fawful as it's the only game he appears but not as an antagonist and it may very well be his last appearance so make it count lol)