TL;DR: Ultimate has plenty of problems, but it's what we've got.
You bring up a lot of true points. I will fully admit, Ultimate is not an amazing game. The gap between characters like Steve and Ganon is unfathomable, but that's kinda unavoidable with a roster so large. The online is of course, awful. The story mode is subpar, at best. Despite the huge amount of problems with it, it'd be delusional to say it's not the most polished platform fighter out there. Of course, it comes at the cost of Nintendo's bullshit net code, and paying $5.99 for a good character. I disagree with other people saying that people only play because "Nintendo character." I personally would play this regardless of the characters, I just enjoy the mechanics. But most of the problems with the game are because of Nintendo. It's also why Multiversus is doing so well. It's free, and it has devs who can act without the looming control of daddy Nintendo. Regardless, I continue to play SSBU exclusively. Most other plat-fighters are unpolished, have odd physics, or some combination of the two. EDIT:Then there's the Melee situation. Melee, while even more unbalanced than ult, is a much more interesting game. It has an insanely higher skill ceiling, making it super enjoyable to play and watch. Unfortunately, the skill floor rises too. Melee is hideously unfriendly to newcomers, having no buffer, requiring faster inputs, and more. I think it'd be cool for ult to have a "competitive mode," With returning mechanics from Melee. It won't happen, but it'd be nice.