Wasn't the issue with custom moves the fact that they were so goddamn impractical to obtain? I mean, the discussions around stuff being op or lame would have gone a lot better if everyone could try these moves for themselves. As it was, we were forced to grind like crazy or jump through a bunch of hoops that involved Smash for 3DS if we wanted to have access to a full custom roster. Who wants to do that when they can actually have fun playing smash? I'd have loved to see a customs meta, but I blame the devs for this one.
This is an important perspective that has to be considered when we look at customs. They created such a massive barrier to entry for new players, and I think this is one of the best reasons justifying their ban. If I remember correctly the best options for getting *all* customs onto your copy of the game were:
A) Going to a tournament or gathering and getting someone else to transfer their entire 3DS of premade customs sets onto your Wii U
B) Dragging together a premade 3DS full of customs for yourself legitimately (taking many, many hours)
C) Dragging together a premade 3DS with hacks (which was a little harder at the time and was/is not a good idea with Nintendo involvement, not to mention being seen by many as "immoral" and risking a voided warranty)
C) Putting Ganondorf in the Angry Birds game mode on the Wii U and just leave your Wii idle while he warlock punches the bomb for hours on end, requiring a taped B button or a 3rd party turbo controller as well as a large amount of time you're willing to let your Wii U sit without you playing it
Of course, only getting customs for the specific character(s) you want to play is less of a hassle, but is still a hassle to be sure.
If I want to learn a competitive game like Overwatch or Counterstrike, I pay for and download the game and I'm ready to go. If I want to learn Smash Bros for Wii U in a world with legalized customs, I have to buy the game, buy + download the DLC (if i really want to save money and know what a tier list is I can skip Lucas and Roy), and then jump through one of the impractical hoops listed above.
After I do all of this, if I want to practice custom moves on my own time I have to go on a discord server or Anther's Ladder to find someone who wants to play customs-on friendlies with me. Customs not only would've hurt the game with the negative reputation and boring viewing experience of custom Villager, but would've created a massive barrier of entry for little good reason.
[All this being said, I still agree that the Miis were neutered by the ruleset for no good reason.]