This is very true, look at Activision as an example. According to wikipedia they have 9 development studios and they used to release a bunch of games each year and now they just pump out 2 games each year (annual Call of Duty+annual kid/family friendly title).

Infinity Ward, Treyarch & Sledgehammer take turns releasing the annual Call of Duty title.

Vicarious Visions, Toys for Bob & Beenox take turns releasing the annual kid/family friendly title (formerly Skylanders, currently Crash/Spyro remakes).

Raven & High Moon used to create original games but now serve as support teams on Call of Duty.

Demonware creates middleware for the other studios to use.

This is true for other major studios as well where the mindset has shifted away from "more games=more money" and now focusing on a handful of big money makers.

Did everyone forget Nintendo was still putting out 3DS games even when the Switch released? I think those games should count.
(Going by NA release along with published) In 2017 Nintendo put out more 3DS games than Switch. In 2018 they put out more Switch games but they still had like 8 or so 3DS games. 2019 they had 3 3DS games.

Yes like Miitopia. Where are games like that besides what people have said...  Arms.  I see they finally announced Brain Training.  Waiting on F-Zero, Pilotwings (even short 3DS version was fun), Advance Wars and any other forgotten IP like maybe Star Fox.

There's Ring Fit Adventure, Sushi Striker was cross-platform, and there's Box Boy + Box Girl and Tetris 99.

If I'm not mistaken Animal Crossing is from the Splatoon team, so that's one big first party team not seen since 2017 accounted for in 2020 already. It's also possible next year we may see BOTW2 (though I personally doubt it) the next 3D Mario, (please be Galaxy 3) the next game from the MK8 team... probably not Monolith though as they seem to be positioning Xenoblade Remastered as their 2020 title, making me think we'll get their new game in 2021.

My guess is that, considering this time they have some third party support, the console still has an overall appealing library and there's no rush to release a game from every IP they have
On top of that, I wouldn't be sure of the existence of a lot of mid-tier Wii/DS similar projects. Iirc, Another Code R for example won't happen, neither Sin & Punishment, 2 Metroid games and a good Trilogy, another Disaster game, Wii play or Wii party.