The main thing that facilitated Ultimate's sales was the Switch's install base. Most series are unlikely to again reach the same commercial heights of their Switch iterations, like Mario Kart, Zelda, and AC.
It's not that they would be super keen to make a bunch of cuts, it's just that continuing to build off Ultimate isn't tenable, and eventually they will have to rebuild the roster. And when they do that, getting close to Ultimate's number just isn't feasible.
As long as the system's install base is healthy, and Smash has its staples and its newcomers (and is still good), the game will sell very well. Pokemon cut like half its dex, greatly angered the online fanbase, and put out a Gen that, frankly, was kind of underwhelming, and it still sold fantastic because it's a super popular IP on a system with a giant audience.
This will be the case with every Smash