I think the idea is just priority. Ultimately everyone will lose a favorite in the transition, but many think that the roster can be cleaned up a bit as 75 characters from roughly 20 years of titles will obviously lead to some less than favorable characters who are just there now. I really don’t have anything against any characters, but there are some clearly added to pad out roster picks and quite a few time sensitive picks.
Ultimate is also on the Switch, which pretty much guarantees high sales. If they brought them in during the Wii U, than the sales would be lower.
To be fair.. Smash isn’t technically a fighting game.
I think another point is originality, as obviously Scorpion will continue to play like Scorpion because MK is the only thing he’s in. He’s not a character based on outside sources translated to a fighting game. As for the other fighting games, usually new mechanics and/ or story will be present. Other fighters also don’t bring back characters only having half a roster of old typically, while Smash’s has only grown to outweigh the new.
You could really apply this to any big change that happens. I’d even say MK8 was in this same hypothetical situation as that game clearly had less characters and modes than MK Wii, yet it was still received and sold well due to the Racing having a big amount of emphasis placed on it as new mechanics were added. Even Smash Wii U was well regarded even though it took out every big mode from Brawl like Subspace and removed 2 of the Trainer Pokémon, Snake, Ice Climbers, Wolf, and Lucas with 13 new playable characters as compensation (2 of which being clones).
I don’t think people realize how well a reboot could work. We may even be able to have a Subspace again as there aren’t 75+ characters that need to work in the levels and have a place in the story. I’d give Smash room to breathe and try new things because right now, there’s too much content to cypher through to really try much outside of “Smash”.
To conclude this tangent, I guess I’ll say my piece. I think a reboot will become necessary at some point as we cannot possibly go on infinitely by just having the same game with a few new characters. At some point a change to the formula must come or there will be a franchise fatigue. I believe Ultimate is the perfect end to the series in its current form as it is the compilation of everything Smash has built up to. It feels like the proper end to an era, leaving the perfect opportunity for a new direction as fans of the old have their conclusion. Smash is like the MCU to me, and Ultimate is the Infinity War/ Endgame. We got a conclusion, but now a new story can be told with some old faces returning and many new ones.