Wrong, Prime 4 and Bayo 3 would aimed at totally different audience than Mario Party, you can bet that Bayo 3 and especially Bayo 3 will have much more attention, marketing and general hype on market than Mario Party.

They might be more niche, but in terms of selling hardware I think they're bigger than Mario Party.

Also I'm not sure Prime 4 is definitely "niche", hell with the online service Nintendo may want to make Prime into their own version of Halo.

Only things we know about or we assume:

-January: NSMBU DX, Travis Strikes Again
-February: Metroid Prime Trilogy
-March: Yoshi Crafted World
-April: Daemon X Machina
-May: Fire Emblem: Three Houses
-June: Animal Crossing
-July:
-August: some Wii U port
-September: Luigis Mansion 3
-October: Town, 2D Zelda or some Zelda remaster
-November: Pokemon gen8
-December: Metroid Prime 4

Plus some 3rd party exclusive/s, Wii U ports, smaller games and multiplatform games. 2019. will most likely be killer year for Switch.

I know it's wishful thinking, but I hope that Animal Crossing releases in March. That's the only way I can see them making the 20 million. And it would be nicely spread out from other system sellers.

If you only cared about how well software would sell systems, then you wouldn't say this looks like a weak lineup at all. Fire Emblem Fates for example sold 1.84 million just 9 months after launch in Japan, one month after launch in America, and TWO MONTHS BEFORE launch in Europe. So by now that's probably at 3.2-3.5 mil. Luigi's Mansion 2 sold more than 5 million on 3DS. Animal Crossing sold 11.78 million units on 3DS. And Pokemon has sold 16.31 and 16.12 million for it's mainline titles on 3DS (though Sun and Moon probably could have sold 18 million if not for the Ultra versions). Those are all huge games, and when you consider Switch has Yokai Watch 4 next year, and that Town will probably be huge in Japan, yeah ... I don't see how next year isn't impressive.

Although, your original post probably meant that to your personal tastes the lineup is weak. Which is fine. 

See post above. Also, I don't know how Animal Crossing being in March would "spread out" system sellers. Yoshi might not be a major system seller, but it does sell some units, and Fire Emblem will probably be a massive system seller in Japan. Also, if NSMBUD performs even half as well as Mario Kart 8 Deluxe it may very well sell hardware (although I doubt it will sell hardware in the same way, nor do I think it will be that successful). Early next year is actually looking pretty crowded for Switch ... I think a summer release would be more spread out actually. Think about it: a late release in July after an announcement of a Switch mini at E3 1.5 months prior. To me, that guarantees more than 20 million ... I think you're underestimating the lineup just a tad

I think that third party game DxM will be a summer game and we have unannounced games coming out later. Maybe like 2 more wiiu ports.

Metroid Prime 4 in Dec would be great