I genuinely think Nintendo, if "software isn't ready" is the reason, would be better off just releasing the thing and playing catch-up than delaying it outright, if in fact they don't already have a sufficient software buffer to make it through this year healthily.
(And I know there are theoretical releases for this year that could fill in the gaps, but those are theoretical. It's just as likely some of those releases are also waiting on the Switch 2 release, or weren't going to be ready this year anyway, or any number of other scenarios).
They'd be pretty brave to bank this hard on MP4. Metroid Prime is bigger than mainline 'Troid, I think, but Metroid as a series is still only a moderate seller.