Switch sales aren't that strong anymore, with sales having been fairly weak everywhere other than Japan for the past 12 months now. Sales aren't crashing or anything, but with no second system to back up the company like they used to have, from a business perspective its gonna be pretty hard for them to keep having falling numbers in quarters again and again and again.
They are gonna want to get the successor out as soon as they can, but they still have some Switch games coming next year which will presumably come out before next gen launches. And the whole reason they pushed it back to 2025 was because their next gen games weren't going to be ready to go for this holiday season. So I don't think a March launch to emulate the Switch is likely. But waiting until late in the year just sets Nintendo up for a really weak first 2.5-3 quarters of the year in 2025 which they are definitely not going to want.
I think most likely we'll see a release right around June, right when kids are getting out of school for the Summer so that you have all these kids starting their break and wanting to spend the Summer playing the brand new Nintendo system.