I have exactly zero faith in the community's ability to actually step back and go "you know what, we've developed the default specials meta enough, let's try out customs now." There's no fair way to incrementally add customs to the game, it's all or nothing. And defaulting to "nothing" cuts out a lot of options for characters. The fact that customs got pushed hard like this after Apex in the first place is, IMO, a miracle.
Would Average Joe have made it to GF without customs? Maybe, maybe not. But you're glossing over the fact that with customs enabled, DK can compete through GF. I doubt that would be the case if he was stuck with defaults. Other characters I've seen compete in a customs-on GF are Dr. Mario, Wii Fit Trainer, and Palutena. (This of course in addition to the usual suspects of Diddy, Sheik, Rosalina, etc.)
Also, I noticed while watching the VOD last night that a lot of people seemed almost uninformed about customs. The commentators thought that Greninja's Shifting Shuriken could be charged just as long as default. (It can't, the move works off of a simple binary tap/hold distinction.) A Villager using Timber Counter landed several hits with the actual tree, both as it grew and by chopping it down, and none of them killed even past 100%. There was a Meta Knight vs. DK set where all 3 matches were on Battlefield, by far DK's best stage to platform cancel his Kong Cyclone. (The Meta Knight still won, which honestly surprised me.) It's true that Kong Cyclone is an excellent combo breaker due to the super armor and windboxes, but if JohnNumbers is able to punish it with WFT of all characters (dem range issues, mmm) then I am decidedly not convinced that it's the overpowered juggernaut it's made out to be. And in a different tournament entirely, the commentators got their description of Mario's Super Jump custom completely wrong.
I mean, if you bust out a full custom set against someone who, for whatever reason, hasn't bothered reading up on them, learning what they do, or practicing against them in general, then no **** you'll probably faceroll just from the surprise factor alone. You can apply the exact same argument to a rarely-used character but that's no excuse to ban the character. And history shows us that we're unwilling to ban overpowered characters, so why are custom specials suddenly under such scrutiny? It seems like the pro-customs crowd is being held to a higher burden of proof.
Finally, you noted elsewhere in your post that the only reason Diddy didn't show up on stream last night is because your area's top Diddy wasn't attending. Okay, let's stipulate that Diddy (and Sheik) still dominate overall in a customs meta. But with customs, you can also give a fighting chance to DK, WFT, Palutena, the list goes on. Probably add Villager and Ganondorf to that list too, plus the rest of whoever's currently high tier to begin with. And given that people living to sometimes insane percents has been a common point of contention with Smash 4, I'm surprised you consider customs allowing kills at 60-70% to be a downside.