Considering the ballot DLC, I'd say plenty of people would be happy enough with just 3 characters and some new stages, as well as the 3DS content in HD.
Look at Mario Kart 8 Deluxe, and then consider that developing a character for a fighting game is far more intensive than one for a racer.
I think you're being overly optimistic of things, and moreover, underestimating the cost in manpower and the like in supporting a game in such a way. That essentially the model for SFV and it's been massively costly for Capcom, EASILY their most costly SF game by far, and the main reason they've kept up with it is cause the game's been a financial disaster, and they got a lot of **** for partial game updates last gen with SFIV. That model, isn't just have a couple of guys working on it. It's have the director and the entire team providing support for the game well after development. Like they did for DLC. And as far as Sakurai is concerned, he's done. He might be convinced to come back to do a final update on 4 for Switch, but he's not gonna stick around and provide yearly support for the game when he's said he wants to work on other stuff.
Moreover, you and Gwen say "finding a new person" like it's easy. But it's not always so simple. Nintendo is very protective of their IP, and Smash is a big name. I reckon they'll pass it off to someone else when they find someone suitable to take up the mantle. Handing the Smash IP to the wrong person could tarnish the franchise. This is why I also say they'd likely wait along time after the Deluxe port, simply to kinda give the series a rest for a while, while the find a new director, and give things enough time to justify a reboot and a new direction.
I'm certain Switch will only get a Deluxe port akin to MK8, and that's basically it. From there, I say expect news on Smash to be very silent for a loooooong time. Like we've seen with other IP in the past. And the main reason being, it's partially cause of Sakurai's health, but also, because they basically went all out with 4 in order to satisfy people. This is Nintendo saying, "this is it, we put as much as we could in terms of resources, manpower, and content, into this game, this is our best attempt to satisfy everyone, if this isn't enough, then you'll never be satisfied." When I look at Smash 4, that's basically what I see. And when I have conversations with other people outside of here on this subject, most people are perfectly fine with a reboot of sorts, or half the roster getting slashed, etc... because they feels Smash 4 is the best as the game could possibly be.
Think of it from that perspective.
That's kinda what Nintendo and Sakurai are telling everyone, and that's kinda how the majority of the gaming public sees it.