If you're wondering why Nintendo hasn't come up with a brand new franchise in a little while, an ex-Nintendo developer may well have an answer for you: it doesn't really need to.
Speaking to Bloomberg, Ken Watanabe, a former Nintendo developer who's worked on games like Pikmin 3 and Splatoon, says new Nintendo franchises haven't appeared "simply because there's no real need to make them".
Rather than creating new franchises, Watanabe says the Japanese gaming giant focuses on "gameplay mechanics first", and then "pick[s] whatever fits that new gameplay best" as a franchise "skin or...wrapper".

Watanabe says that Nintendo's focus is on "creating a new way to play", and that the studio will pick the franchise that it thinks most applies to that "new way".
I'd argue this design approach is evident in this year's rather excellent Donkey Kong Bananza; the "new way" Nintendo found this time was to create a 3D platformer full of destructible terrain, and Donkey Kong best fit the idea of wanton destruction mixed with 3D platforming.
According to Watanabe, it's also common for Nintendo staffers to "secretly work on something without telling their boss", and if that idea is interesting, it "gets turned into a real product". He describes making a level in Pikmin 3 in his spare time, only for Nintendo to include said level in the final product.
Artist and designer Takaya Imamura follows up by saying that Nintendo "lets [its staff] just focus on building what [they] believe to be fun, without distraction". That would go some way towards explaining the generally high standard of Nintendo's work.

This philosophy is also presumably why the recently-revealed Splatoon Raiders is a Splatoon spinoff rather than a game belonging to an entirely new franchise, although exactly what shape the gameplay in Splatoon Raiders will take remains to be seen.
In any case, it does look like Watanabe's idea of how Nintendo operates hasn't done the company any harm at all; the Switch 2 has sold very well indeed thus far, so whatever Nintendo is doing, it's likely to continue into the near future, at the very least.