The notion of Smash Switch being some sort of content-light port of Smash 3DS/Wii U has always been implausible*, and whilst it briefly had a small amount of plausibility at the start of 2017, that time is long-gone, now, I feel.
*The fact that its source was the same group who brought us "Splatoon The Enhanced Port Definitely Not A Sequel as launch-day Switch game", "Two Switch SKUs at launch, one with more memory and Splatoon The Enhanced Port Definitely Not A Sequel as a pack-in game", "Zelda delayed until later in the summer, 3D Mario as big Switch launch-day game", "Switch only available in grey at launch, and then white later", "Multiple individuals confirmed that Mario Kart 8 Deluxe has new cups", and more, doesn't help its case. "Ballot-winner Shovel Knight" is also a blood-relative of these rumours. We all know how each and every one of these turned out.
Reason being, we know that they started work on the next Smash game in April of 2014, according to Bandai-Namco's own "Smash 6" job-
listing (bearing in mind that Smash 3DS is
considered "Smash 4", and Smash Wii U is considered "Smash 5"). That's the same time that work on Super Mario Odyssey
began, along
with Mario + Rabbids Kingdom Battle. It was a year-and-a-half
before Splatoon 2
started development, too, since that was made in only two years. This even lines up with the timeframes that were claimed in the Diddy Kong Racing 2
rumours.
Folks often cite the case of Mario Kart 8/Mario Kart 8 Deluxe here, but I think that folks might not be looking at this from all angles - when Mario Kart 8 was released on the Wii U, it was clearly unfinished, and it had a number of presentation issues and bugs, some of which were never fixed in that version of the game. Moreover, it had some interface design decisions that were downright boneheaded for the Wii U, such as using a vertical split-screen for two-player games, which made perfect sense only once the Switch came into the picture, with its stock controllers attached to its sides - the sides that each player gets in a two-player game. If you ask me, Mario Kart 8 wasn't ported to the Switch - it was backported to try to keep the Wii U afloat.
The case of Pokken Tournament DX doesn't apply here, either, as Pokken Tournament was designed from the start to have both home and arcade versions, and both get used in official events. Pokken DX simply continues that by providing the home version on a current machine.
And that's not even touching on the fact that a lot of content and references in Smash 3DS/Wii U are extremely out-of-date by now, with a number of them having been quite old in the first place due to all of the recycled content from Brawl. To try to sell that again in 2018 seems like a very odd idea, and considering that Smash entries often tend to be a bit of a time-capsule relating to the state of things when they were released, it wouldn't reflect the system that it's on very well at all.
To me, trying to sell a minimally-altered port of a three-and-a-half-year-old game as what would be one of the Switch's hugest offerings for 2018, which is very likely going to have to act as a flagship title for the soon-to-be-paid online service (something it can't do well if you can just play a near-identical game on the free-of-charge 3DS/Wii U online services), sounds like a pretty strange proposition, and it simply doesn't take four years of development-time to do that.
EDIT: And just look at how they're going all out for Animal Crossing: Pocket Camp - a promotional mobile phone/tablet game. Why go all-out on that, and not on a major release for the console that they're trying to use such things to promote?
