I don't think assist trophies would necessarily be locked out if the next Smash game is just Ultimate Deluxe. Mario Kart 8 Deluxe removed content from the Wii U version that didn't make sense to keep, like the Miiverse stickers and the battle courses. I could see them doing something similar if they wanted to upgrade Waluigi, Spring Man, or Issac in a hypothetical deluxe version.
Honestly, the more I think on it, giving Ultimate the deluxe treatment as the next Smash game may be the thing that makes the most people happy. It seems like almost everyone is in agreement that, unfortunately, there really is no way to top Ultimate in terms of scale. So, why bother trying? Why not just continue to ride the wave as long as possible? Some people might be pissed that the Switch 2 doesn't have a unique Smash game, but that'll probably in the face of such a celebrated game getting a second run with new content. Plus, we'd likely get Smash as a first year title, as opposed to the second/third year title it's often been in the past.
As the technical advances of each console generation continue to shrink to smaller and smaller differences, people seem to be more and more alright with ports from previous generations. If Sony was confident enough to have GTA V, a 7 year-old game from two console generations ago, as the headliner of their PS5 event, I think publishers are aware of the value of having older, established products available on new systems. Hell, the best selling Switch game by a good margin is Mario Kart 8 Deluxe. It's a win on both the business front and for keeping the fans happy.
The only issue is, obviously, licensing. There's a lot of hands in Ultimate's cookie pot and it's going to be difficult to get all parties back on board. That being said, I don't think it's necessarily impossible. I think most corporations realize that licensing a character into Smash is basically free money and good publicity at this point. Even Konami and Square, the two companies most speculated to be the problem children of rejoining Smash, have given a lot of content to Smash at this point. Even if worst comes to worst and some IPs fall through the cracks and have to be removed, things should go over fine as long as they don't have a full playable character. Like, it would suck to lose Kunio's spirit or Vault Boy's costume, but it would be a relatively minor complaint to Banjo, Cloud, and Snake returning.
The only major issue is Sugiyama potentially trying to suck up 20% of the game's sales because he had the generosity to grace us heathens with 8 midis, but that's still a relatively minor speed bump in the grand scheme things.