If people kill early, it's janky. If people don't kill early, the game is slow and campy. People will john about the game no matter what is actually in it. The kill percentage of a move like Dark Fists is perfectly fair and natural; it kills early but it's a big move. If you hit with a big move, shouldn't you kill early? I also don't think looking at characters as a "base" and customs as an "addition" is a reasonable way to look at it. It's more that the customs are a part of the character's toolset and you look at it as a whole. None of the characters you listed are problematically good in the custom meta; the best character is still Sheik, and while Rosa is probably #2, she actually hard counters FEWER characters (there are so many customs that seem designed to give Luma a middle finger). The fact that you implicitly are admitting that however bad these customs are they apparently don't have such power as to really change who is making top 8 kinda says a lot; at worst they're obnoxious (which is purely subjective). If they were broken, they would by definition be making an impact in this way.
That's not really the point. A move killing at obscenely early percents is pretty much universally agreed to be a negative effect. Can anyone really look at Soaring Tornado kill someone by the ledge at 30% and say it is a healthy move? No, we all know that this kind of early kill is not something that belongs in the natural flow of the game. Are there non-customs that can kill early? Sure. But we have a lot more power with custom moves and can actually strip away those types of early kill options for a healthier metagame. It doesn't matter if characters are problematically good in a customs metagame. It's actually not really that relevant. A move doesn't have to make a character overpowered for it to be banworthy. There's such thing as overpowered/overcentralizing/janky custom moves despite what character they are on and those are all banworthy traits, at least in my opinion. To me, results are relevant in this regard, but they don't tell the whole story. The obvious truth is that having a banworthy custom move doesn't automatically make you a god at the game. It does increase the probability that you may be able to cheese a win out of better players, but you still aren't getting top level fundamentals or reaction time, and that's what it takes to break into Top 8 or even say Top 16 at a supermajor like EVO. That's mostly because top players aren't Customsmaster72, but are going to stick to their default metagame characters because it would be silly for them not to do so. I do think we will be seeing some randomly high placings from certain players who use characters that are reliant on customs, but it's a little absurd to expect and use Top 8 as an all-inclusive boundary for what customs we can actually judge as being banworthy.
I don't buy this idea that customs are inherently part of the competitive game. You can turn them off; they're off by default. Customs haven't really been balanced in patches. They have to be unlocked and it's not that easy to do. There's way more evidence pointing toward customs not being intended as a competitive mechanic than the opposite.
As per the community, the last time we polled on it, we found 80% of the community supported customs. To be frank, I don't think there's any possible way we can as a community credibly go the way 20% of players want when 80% want something else regardless of who those 20% are. Top players are defined by the game and not the inverse, and we shouldn't change the game to match the preferences of a minority of the community. I also don't recall dabuz being against customs; he was pro-custom the last time I checked (but I don't stalk him online so I dunno what he's said lately). In general I think you'll find less animosity toward customs among strong players than you are projecting, and even if you didn't, I think you're just vastly understating the massive public support custom moves have (there are probably lots of pro-custom people even in places like Europe and Florida even; you have to ask all of the people and not just the few most vocal/powerful people from those areas). I want to play the best game and the game that the community really wants. That's the custom game.
First of all, it's really difficult to take public polls seriously. The reason for this is that a lot of people voting are not the ones attending events or the ones hosting events. There's nothing to qualify opinions. Also, that poll was a LONG time ago and I think there's been a major overall change in community sentiment. There was very little community opposition at the start, even from top players. Customs just seemed interesting and cool for the most part and we went for them. Now that we've had tournaments with customs, there's been less and less overall support for customs and a bunch of different concerns that have been brought up. A large amount of top players are solidly anti-customs at this point, and it isn't just them. There are definitely regions that are running customs events just for EVO, almost forced to because their players need to be prepared. I suspect we'll being seeing a lot fewer customs tournaments as we go past EVO into Super Smash Con, TBH5, and the next Apex (all current evidence points toward Apex being non-customs). I feel as if the actual support for customs is much lower than you think and much more based around the fact that EVO is running customs. Even regions that have customs ingrained will I'm sure be running more non-customs tournaments to prepare for the upcoming supermajors. They'd be foolish not to run at least some non-customs tournaments, to be quite honest.
One more general note about community polls. It's really difficult to take Smashboards community polls and even general community polls that seriously. The current poll about whether to ban Sheik + G&W in EVO doubles would have the strategy kept legal when pretty much any right-minded competitive player looks at that strategy and sees it as a broken/overpowered team. This isn't a democracy and that's why we've had the back rooms for previous games. The Smashboards community is oddly liberal in general; nowhere outside of Smashboards will you see anyone take making a stage like Orbital Gate Assault legal seriously. In most mediums/areas of discussion, there's more debate about whether we should even keep all the stages on our current list or maybe add some stages like Wuhu or Mario Circuit, but there's almost no regions that are running expanded stage lists or even considering them at this point (key words: I said almost no, there are some outliers). Things to make more stuff legal, pretty much with anything unless it's really beyond beyond ridiculous, pretty much always have support that doesn't exist anywhere outside of this forum. Overall, I think polls here represent a slanted portrayal of overall public opinion and aren't necessarily effective to be used in the creation of our ruleset. Sort of like how community-voted tier lists usually kinda blow and have to be taken with a significant grain of salt, which is kinda proof of concept in this case.
To address what you said about the health/survival of the game: there's nothing to show that customs are vital to this. The non-customs metagame is pretty remarkably balanced as far as Smash games go. We don't really have much of anything that's super OP or janky or what have you. The default metagame is deep and interesting, and we've seen some really intense competition in it. CEO is proof of this; the game's first supermajor since January, running no customs, turns out to be extremely hype and well-received by pretty much everyone. I think we'll really have to wait until after EVO to pass the ultimate judgment on customs, but I think it's reasonable to say that there's going to be more overall negativity attached to EVO by nature of the ruleset because I'm sure we'll be seeing Wind Kong, customs Rosalina, and customs Pikachu at the least in full force...their customs are not healthy, bottom line.
I also will point out that, if you want unity, suggesting the custom side should just surrender isn't really realistic at this point. We're way past critical mass. I know my scene is never going back no matter what happens nationally (our main TO is not going to budge on this), and I am pretty sure plenty of other scenes are deeply entrenched too. At some point, you're going to have to budge some very stubborn people to make unity either way, and I'm not really convinced customs off is going to be an easier way to succeed at that.
I'd more say that it's not realistic to expect customs to ever become universal. To be completely honest, I think it will never happen. Never. Most of the world outside of the US doesn't run customs, including a region that is likely just as powerful as the US (Japan). I'm pretty sure that Japan will never run customs; their players don't really have respect for customs as a whole. Most other areas internationally don't show any signs of changing anytime soon. Customs turn off a lot of top players. Sure top players are defined by the game, but it's not good if we're running a ruleset that a lot of our top players are opposed to and in many cases don't really consider legitimate. Customs aren't way past critical mass, heck critical mass wouldn't even matter because the TOs of supermajors and top players/community leaders with a lot of influence are expressing this anti-customs sentiment and they're the ones who you need to convince. You're not pressuring anyone to subscribe to a non-customs metagame; it doesn't really work that way. I don't think universalizing customs is possible. Either we have a split or no customs. I might be proven wrong, but that's what the big tournaments are showing for the future so far.