I'm guessing it'll be higher than 20 million, but not much more than that. So I went with the 19-20.9 million option.
The Switch is still going to get a plethora of new games, big and small, most of which I think will be 3rd party. But as far as Mario is concerned, B-C tier spinoffs like another Mario Party, Mario Golf, Mario Sports Mix, or another Mario RPG (Whether it's a new Paper Mario, Mario & Luigi, something new, etc.) is all we're going to get. I think the big, blockbuster, A-S tier stuff like Mario Kart and the platformers are being saved for Switch 2. I fully expect Mario Kart 9 to be a launch title and the next 3D Mario game within the first year of its cycle. Once Switch 2 or whatever their next system is comes around, THEN I think we might see a brand new 2D Mario game... Assuming they haven't just replaced the 2D line with Mario Maker.
I agree with all you said...except the bit about no new 2D Mario on Switch. MK9 Switch 2 launch game for sure. With 3D all-stars and 3D World and maybe a separate Galaxy 2 release at some point (??) I don't think a second new 3D Mario with hit Switch. But for 2D all we have is an old port that certainly did not sell like a normal 2D Mario game, as compared to MK8D which was clearly accepted as THE Mario Kart game for the Switch and thus why we won't be seeing another one. They also slipped out NSMBU:D as a january release as just some post-holiday filler. They are no doubt planning a new 2D Mario with all the marketing and timing that goes with some a major release. And yeah I'm sure we'll get another Mario Party and some assortment of Golf, Sports mix, etc on the Switch in the future. But it'd be truly bizarre for them to skip a new 2D Mario game on the Switch. That'd be a major game and a 20+ million seller for sure, and with just Mario Maker (which is a different kind of experience) and an old port they certainly haven't hit that spot in the library yet. 100% chance Nintendo is bringing 2D Mario to the Switch in the next two years.