Well, if someone can get really good at one character, then they would then be able to clear out people who are only good with a few characters - it's not like all 3 on a team attack at once. An amazing Shiek can still beat out an ok Diddy, and then an ok Cloud, and then an ok Bayo. Then it becomes a meta choice about how to spend your time training, and the end result is still more interesting play. There was no need to really do anything but one character in 4 - although people still did it to a degree - and so that leads to people not putting the time in elsewhere. And if there's the easy character, so what? People can just do that same thing anyway with an all singles lineup. As it stands now, there are no pros in Ultimate because the game hasn't come out yet, so it seems like a foregone conclusion to avoid what would be a vastly more interesting viewing experience just because it clashes with how it worked in the old game. I think it's jumping the gun to assume that players can only have a finite amount of skill in a game and using that as justification to avoid more variety. Plus, I don't think "it's harder for the pros" is very defensible either; I think that this just ups the skill ceiling.
I think it would be the exact opposite. I think that if there was SS, there'd be less easy top tiers and more niche. For one, a top tier would be less likely to be able to carry an entire fight, so someone couldn't use an easy character as a crutch the entire time. Yes, you could point to the example above, when I said that an amazing character can carry, but that's against someone who is only ok with the others - they'd be just as much a carry if it was an amazing Shiek vs an ok Diddy for 3 stocks too. At some point, between 2 players of equal skill, I'd say that it's not too unbelievable to think that someone loses a stock, and thus there goes their carry. For two, there would be more of a chance for unique built-in counterpicks, because people will know that they're fighting against more than one character, and don't need to have a specific counterpick carry an entire set of stocks. The meta instantly becomes deeper and goes beyond the application of a single character and where they fit into the tier list. It no longer becomes a question of "how does this one character rank" but moreso "how does this team work together".
Players would still pick top tiers in SS, sure. But they'd also pick more non top tiers, because there is less riding on any individual pick, which means more chance to experiment and pull a pocket in response to a lineup. I'd think that if someone picked PT, they'd also be assured in knowing that their opponent would need to know an extra 2 fights too (aka, the same thing that is the case with 1v1 and picking PT). Moreover, why wouldn't we want people to have to learn more characters? It's not a skill thing, since that assumes that people can only get so good, which I simply don't believe, and is a viewpoint predicated on a system which the Smash community has never been exposed to. Wouldn't it be great if people knew how to play more than just the one, and we could all wonder how they were going to construct their lineups? Aren't surprise picks super hype? Imagine that sort of selection mixup being built into the meta from the very top. This is on top of the unquestionable mixups in matchup variety too. End result: much more interesting.
The only counter-arguments seem to be based on a lot of assumptions for things we have no idea about, and that the older ways are better just because they're older - and ignores the objective facts of a greater matchup variety and deeper meta.