While I do love Ultimate and I think an Ultimate Deluxe would be a great opportunity to supplement a lot of my more minor complaints with that game, I've also put 350+ hours into Ultimate over these past 3 years. I've got every spirit (minus the Chozo Soldier, which I believe is still not in rotation for whatever reason), I've completed every character's classic mode, and I've beat WoL three times now. I have no idea how many versus matches I have played since launch, but I believe the number is floating around 2000.
My point is, I feel as though I've really got the full experience of Ultimate at this point and, honestly, I certainly would not be opposed to something new in the future. Even if Ultimate is revived for the next console, what happens after that? All good things must come to an end eventually and I don't think 3 years of DLC support and 25 million copies sold is the worst place in the world to stop.
I'd be thrilled to see more characters, stages, and modes in an Ultimate Deluxe, but I also don't want Smash Bros. to become Grand Theft Auto and constantly rerelease the same game over and over in favour of developing something new. I'm more than willing to make peace with the idea that the next Smash will not have 89+ characters so long as it is actually the next Smash.
I'd also like to see some of the cast get updated tools and some of Ultimate's mechanics changed (namely, the way movement on platforms is handled), which may be beyond the scope of a simple rerelease. However, Mario Kart 8 Deluxe also changed and added mechanics, so I guess I can't say I'd be inherently opposed to an Ultimate Deluxe for not being able to make those changes until I see that Ultimate Deluxe can't make those changes.