Just because Bomberman was Mii Costumed, doesn't mean the idea of promotions are dead. There are still other Potential characters like Waluigi, Shadow, Isaac, Shovel Knight that have their own merits to be Promoted
Selling less than Ultimate has is a non-issue because it's sold so absurdly well in it's own right that they shouldn't expect to come close to it in anyway
It could be an issue if perhaps the cost of all the licensing + all the added content really adds up
A Hypothetical Smash port doesn't have to sell exactly as much as Ultimate, but it needs to be around the same ballpark. A Port selling perhaps only 1/4 of the copies of Ultimate would not be good, because with all the licensing Nintendo has to pay, along with the development costs, it's likely they won't make much of a profit as if they just perhaps retooled the game in it's own right
The issue also with comparing MK8 and MK8D to a Hypothetical Smash port is that the original Mario Kart 8 was on Wii U, which was one of the worst selling Nintendo Systems. Mario Kart 8 Deluxe works so well because for a Large majority of people, this is their first experience to Mario Kart.
For Smash, 20 Million people are already exposed to this game thanks to the Switch ecosystem. A Simple port won't cut it. What they could do is Rebrand the game as something entiringly new with perhaps different Selling Points (Like maybe over 100 characters), along with added modes like Story (with Platforming), battle royale modes, Customization, etc.
Of course it's that balance of the development costs, amount of licensing, and potential profit.
If there is a port though I imagine there's a plan already in place for it. Perhaps the reason there is no Fighter Pass 3 it's a much better idea to just make an entirely different game. There's probably negotiations going on right now for the future Smash game. That also could perhaps be what the Apex Rumor or League rumors lead too if they aren't playable in this Pass