I'm not so sure that "smush" is going to be the capstone you're describing. Of course, details are slim and hype is high right now, but I think there are a few factors to keep in mind. As a competitive melee player, Nintendo has proven themselves to be incredibly slow and remains out of date with regard to supporting a competitive community for any game they have. They do seem to be trying to turn a new leaf with Ultimate, but Nintendo Japan has a lot of internal inertia (Sakurai's personal opinions) and regulatory restrictions (gambling laws making it hard to host tournaments with meaningful prize money) that make me skeptical that they're going to go as far to support their game as Valve, Riot, Blizzard, etc, which is what's really necessary to become a tier 1 e-sport. These leading e-sport companies are all American, in contrast to Nintendo which has to work overseas to support smash competitively (the Japanese competitive smash community is tiny compared to the U.S., in part due to culture).
In addition, I don't think Nintendo's going to be able to kill melee and convert all of us to ultimate without also turning off lots of casuals. But without the full support of both casuals and the entire existing competitive community for smash (over half of which is for melee), ultimate is just going to be another profitable and fun game, like smash 4, but not lastingly legendary like melee happened to be.