I'd go so far as to say it's inevitable.
Ultimate is in a unique situation in that they were actively planning for it in the post release of Smash For. Nintendo knew the Wii U was a sinking ship and wanted to have a killer new Smash to boost Switch sales in the earlier part of its cycle. This likely went into their character negotiations for characters like Cloud and Ryu, and contributed to the effort put into bringing back Mewtwo, Roy and Lucas as DLC. Ultimate, in many ways, could be seen as an eight year project from Smash 4's development beginning to the end of DLC support for Ultimate. It's been the same team and more or less a continuous project.
The next Smash won't be that. They're going to wait until their next console, then assemble a new dev team, with or without Sakurai, who will need to start from scratch. Even if they use the same game engine and port things up, trying to renegotiate the rights for all these characters and stages and music, etc., will be a nightmare and, after Ultimate's immense success, all of the third parties will want more than they got last time. Nintendo won't see it as worth it.
I genuinely believe that the next Smash will be a reimagining of sorts, trimming the roster down and reworking the base mechanics to make a brand new kind of Smash experience. And then Ultimate will become the new Melee, because it just won't die.