If Ryu is already irrelevant (wait, when did that happen? I just saw him take a finals in Doubles like a week ago), then further harming his relevance is literally no different than harming the relevance of any other character.
Look, if we really don't want to let anything interfere with the static nature of our tier list, we really should be on a single-stage meta with no customs. This removes the greatest number of variables while still leaving in other nonsense like percent as a tiebreaker.
Basically, the "____ gains nothing while others move, and thus ____ gets worse by comparison" argument makes no sense when you compare it to the rest of the ruleset. We could ban Smashville and since Sheik and Ness lose their benefit while others gain an improved matchup against Sheik, it would (by this argument) directly harm Sheik's viability. This can be taken as a true statement, because literally any ruleset change will affect viability across the board.
The goal of improving balance is noble, but quite frankly idealistic, and to use player-made balance would essentially entail coming up with a required moveset, plus stat tweaks via equipment, rather than "here are some cool movesets you can use (or bring your own), but equipment is random and lame.
The purpose of customs should be to enable the more varied game and then let these new tiers fall where they will.
Default cape is faster but completely outclassed, last I checked.
That's not a choice, that's just picking what's good for the matchup. There's almost always clear winners for any given matchup and when there is a choice it doesn't make any real difference. That's a false choice with no depth.
Yes, customizations are cheating. Custom moves and equipment. Why should we use them at all?
Gust Cape is only a pure upgrade if you're against some scrub who gets so heavily mindgamed by the existence of windboxes that they can't function. Regular cape is vastly superior as a reflector, and even Shocking Cape gives Mario a guaranteed kill setup from dthrow, if I read correctly. Gust Cape is, essentially, only so much better at edgeguarding. Even there, FLUDD is just as (or more) functional as a windbox, and the delay in the cape's startup compared to default makes it at best an unfamiliarity mixup and at worst, harder to land as a reversal edgeguard.
If picking what's good for the matchup constituted false choice with no depth, I feel like Ally would have probably run some of those "completely outclassing" Marth customs at EVO. Or maybe it really is a choice, influenced by preference, experience, and one's attitude towards customs.
Because his statement only applies to the combination of custom moves and equipment, and not either one on their own without the other?
Also, since when do we care what Sakurai thinks when building our ruleset?
We only do so when we think it relevant to our arguments.
Aside from that one guy with a green-haired Fire Emblem character avatar who actually thinks a near-Glory ruleset would be interesting and perhaps even enjoyable.