Really, Sakurai would never go with an Ultimate Deluxe. Nintendo could absolutely greenlight one... but it wouldn't have Sakurai at the helm because he doesn't like porting it over. He might still be asked for advice.
However, it also would be under someone who has different ideas of where the direction would go, which would be pretty damn risky to take, which means the chances of having problems increase. Why would Nintendo want that? There's not a very high chance of it, overall. It's not impossible, but there's much more reason to believe otherwise.
Incidentally, Smash isn't Sakurai's baby where he can do what he wants. He's hired to work on it. He also has been with it so long that companies in Japan prefer to avoid hiring a new person as Director because it could make things go a direction that would, as noted, be too risky. That's not a good idea. It's one thing for a spin-off. But a main entry? Huuuuuge risk. Why do you think Nintendo gives Sakurai so much control? They still have some, but for the most part, he takes the helm because they feel he's the best option for it and really knows what he's doing.
Besides that, this is a lot harder to create than we have for Mario Kart 8 Deluxe. It has a lot easier assets to work with, and a lot less balance needed due to tons of gameplay clones with much more minor tweaks. It's like if all the newcomers in Ultimate were Echoes, and even less content than we got. Notice how we barely got any new stages, even with DLC, overall? That's because it requires a lot more work. This isn't even noting that Sakurai made the returning stages from scratch too(no reason is clear, mind you. It's one thing that not all returned, due to the Stage Morph issue or other things like a 3DS-specific stage, but not easily reusing assets does seem odd). That said, even if they were more ported over instead, it's still a crapload of work. 8 Deluxe's extra DLC? They actually had Mario Kart Tour to also borrow from. That's why it worked. Smash doesn't have this option. Literally all it has left is previous other Stages to... remake new, since they're too outdated anyway. So of course it has less options than what are the equivalent of Echoes and easy to port over stuff. That's not the option Smash Ultimate actually has.