Hmmmm not really...
Super Mario Odyssey took four years of development (the same amount of time that BotW took). Tears of the Kingdom is coming 6 years and 2 months after BotW. Just going by this we'd have 3D Mario by late 2023 or early 2024. BUT, that doesn't mean either had 6 years of development (BotW only started development January 2013, despite Skyward Sword being released late 2011).
Apparently Tears of the Kingdom had a hiring call early-mid 2018, with big hiring calls/Monolith hiring in 2019 around E3 announcement. 3D Action game by EPD Tokyo had hiring calls late 2019. That doesn't mean it wasn't already in development with a smaller team, probably since 2018 like TotK even.
Then COVID happened, they moved to a new building, apparently they're moving from their own engine to LunchPak, and Switch games taking longer than ever to make. That alone would have made the patterns/easy guess(at the time) of a late 2021 release impossible even if development started right after Odyssey finished.
Add in a possible change in scope(I guess TotK had one too, maybe when announced it was supposed to be way more iterative and the scope highly increased to a thing bigger than the original), maybe they've gone from a Galaxy-esque sequel to a totally new kind of 3D Mario, etc.
Some dev cycles to compare:
- Metroid Dread: 4 years;
- Tears of the Kingdom: at least 5 years, can be even more;
- Splatoon 3: 4 years maybe? But it came out more than 5 years after 2, while 2 came out 2 years and 2 months after 1;
- Mario + Rabbits: Sparks of Hope: does anybody know? What we know is that it came out 5 years after Kingdom Battle, I'm pretty sure it took longer to make(IIRC from interviews we know M+R:KB started ~2014?)
- Metroid Prime 4: ...
There's those possibilities for the next 3D Mario:
- Only started full development early-mid 2019, right before those hiring calls came;
- Something started development right after Odyssey and got canned in 2019(turned into Bowser's Fury), development of the current game started in 2019;
- The team took late 2017-early 2019 to prototyping and pre-production, full development started by then;
- Full development began late 2018(Zelda began early 2018), meaning it's probably coming late 2023-early 2024;
- A long ass development cycle actually happened, late 2017-late 2023. Unheard of, but development times just keep getting longer, apparently Metroid Prime 4 will have it when all is said and done, if you believe TotK started development early 2017 it's happening with Zelda, so...
With the sole exception of the last possibility, which could've brought something by late 2021-early 2022(like I imagine Zelda would be before COVID and change in scope), it's perfectly normal considering both COVID delaying things and games of that caliber taking longer than ever to make.