I wonder, do custom moves being introduced into the meta become more likely or less likely as the meta goes on?
One argument is 'there are too many matchups,' which one would assume becomes less of a problem the longer players are exposed to such matchups. If we play the game long enough that every existing matchup becomes knowledgable enough that things like Shulk's Wind Monado stop boggling the minds of announcers, is it safe to add custom moves then?
Would it be a valid approach to add a smaller handful of custom moves to the current meta at a time? If players are so inept that they can't learn every custom move at once, would adding one custom set per character at a time help ease their understanding of them?
The same could be said for Miis. Trollish "too many matchups" defenses are null if we add so little at a time that a baby could grasp it. Or would this still be too difficult for master players to pick up?
All experience with Smash's community says "No, they become more entrenched in their conservative moveset, and anything removed will never return."
Which is why the sheer scrubbery following EVO'15 was so frustrating.
Any way, to the "Good characters get good customs so increased viability is offset by moving the goalpost" sort of arguments, here's some analysis from both observation and experience.
1: Most people think Sheik's "good" custom is Gravity Grenade. I agree, it's an excellent tool. But in Smash4 in general, Grenade variants of all forms are exceptionally rare in both default and non, as they're rather low reward save on a pretty hard read, and that hard read may be better rewarded with Bouncing Fish or just more f-airs. Even in EVO, by which point default (or Gravity) Grenade was well-known by customs players as a functional and long-range trick to keep Villager from planking, it was ignored in favor of... attempting Bouncing Fish and Fair against a ledge-camp strategy. Ignorance at its finest is probably the only reason Sheik had issues in that matchup, and Villager the worse for it now with the EBT nerfs.
2: The other "good" Sheik custom is a needle that does more shield damage and goes through (trivially) Luma and the Castle Siege statues (increasingly banned in tournaments anyway). It trades this for, to my knowledge, its ability to true combo into Bouncing Fish offstage for a kill.
3: Rosalina gets a serviceable projectile that makes a handful of already miserable matchups trivially more difficult. It also locks Luma into place for a bit, leaving more punish room than Star Bits.
4: Pikachu's infinite from Thunder Wave is long gone leaving him with a range tradeoff for a punish at high percents. Heavy Skull Bash emphasizes how utterly garbage the default is, but even at its best, it's punishable, can get you killed against windboxes/deflections (Mario's still pretty popular by the way), and in general, turns a useless move into a useful move. To this day it's the only move I think even warrants a thought towards banning, but I've yet to see it prove problematic since valid data sources universally decided they prefer a bland-er game.
5: Zamus's customs are pretty mediocre.
6: Mario can do the same things with or without, at roughly the same difficulty.
Basically, the only goalpost argument I can see with validity is Rosalina's gatekeeper status, and of that, I would like significantly more substantial proof before we suddenly and mysteriously start factoring gatekeepers into our rulesets. Else, I would like to introduce Default Sheik as a gatekeeper against about 50 characters.