A new 2D Mario at that point would be better as a release title for Switch's Successor, or at the very least, a cross-gen game.

Now would be the perfect time to launch a new 2D Mario for Switch.  Super Mario Bros 3 on the NES released only a year and a half before the SNES launched (US) and it went on to become the biggest selling non-bundled game on the platform.

I could see the argument for Prime 4 being a cross-gen game, but a new 2D Mario would be perfectly suited for Switch.

I don't think 2D mario will really move the needle for new hardware unless Nintendo's goes in a whole new art direction. I think its really just the sort of software to capitalise of off the Switch's massive userbase.

Fire Emblem: Engage, 2D Kirby, Octopath Traveller 2, Bayonetta Origins and possibly Advance Wars reboot. A Mario game that ties into the movie is also still a possibility. Could be some 2/2.5D platformer, 3D platformer or maybe even an adventure game or rpg. 

Another 4 million this year is definitely doable. I'm also not convinced Nintendo is done revising the Switch. I think we'll see another version of the console release this year with a new Nintendo releasing in early 2025. 

I think Nintendo should save 2D Mario and all their other big guns for the next Switch.

I think a 2d mario would be a great finisher for the switch
And their big mario odd/galaxy game should be a launch title for their next console

Obviously 2D Mario needs to be on Switch. It'd be insane for them to not capitalize on the massive Switch userbase with 2D Mario. Switch has plenty of life left in it so there is no reason the only 2D Mario on the system should be a last-gen port of a pretty dang mediocre game, that was released kinda as an afterthought after the holidays one year with little fanfare and still easily went over 10m. Even if it comes out only 12 months before the next system it'll probably sell 15-20m. And anyway presumably we'll be getting a 3D Mario at launch of the next system.

2D Mario would make a perfect holiday title this year, though it would also be really cool if they were keeping it under wraps until the last minute and soon announced its release to coincide with the Mario movie release. That would make a lot of sense too, though I feel like they would have announced it by now if it was coming along with the movie in April.

As long as it's not another entry in the 'New' series. Making 2D Mario perhaps the last mega-selling Switch game to release would be a fine way to start a new 2D Mario series.

Since Minecraft is now at over 3 million I had to Update this.

Top Selling Switch Games  according to Famitsu stimated Physical sales charts.

Last time i saw it on the charts top 30 Kirby Star Allies was at .99 and Kirby and the Forgotten Land was at .98 very close but not quite there yet. Im only going by what shows in the top 30 so there is a chance that Star Allies is there already but i can't say for sure. Since you mention the ones that are close lest add a few more