Personally there are only three reasons why I see anybody getting cut:
-Licensing: Mostly for the 3rd Party characters. HOWEVER I feel like if these are the only reasons a character to be cut, then I don't see the other companies being so willing to remove their characters. Maybe they become more resistant on adding more of their characters, but if in Smash 6 or 7 if all the characters return except for Snake and the Belmonts for example, people would complain about Konami's doings.
-Hardware: Seeing as some of the previous cuts were due to limitation of the 3DS, it is a possibility that the console after the switch might not be able to handle some of the characters programming. Less likely because it seems that Nintendo might only be doing on system at a time for now.
-Time Restrictions: The most likely reason. Characters end up on the cutting room floor. We see this with the forbidden seven in Brawl. If the next Smash is given a deadline and they can't reach that deadline, I assume that the lower priority characters will end up being dropped. Highest priorities would probably be the bigger characters like Mario, Link, Pikachu, Inkling, etc as well as the 3rd Parties (because they already went trough the trouble of getting the rights to them, so they better take advantage of it)