I mean, I think we are missing the point here.
First, ACoC is not the "panacea". Haki itself is. Haki became this the moment Oda got tired of thinking clever ways Luffy had to find to fight Logia users who were always pretty much invincible otherwise, even when fighting other logias. water for Croco, rubber for enel, DF hierarchy, Teach's darkness, all of these were too much restricted.
Here comes Haki to save the day.
Also, people are really, REALLY getting ahead of themselves with this "everyone will get it". Only TWO SHs have it, with a strong INDICATION that a 3rd might. But ZERO indication that any of the others will get it. That is FINE. compared to the amount of character in the OP world, having even 20 characters with it EoS would still be a SEVERELY small sample of people.
Also, take into consideration Skypie and Enel.
Enel viewed himself a God, why? Because of his invincibility due to his logia ability. The moment some random took that invincibility away, Enel got rekt. But if it weren't for a "counter", Enel would have destroyed everyone and everything there.
The immortality here serves this very same purpose. These characters view themselves as GODS. This immortality is what makes them a formidable force that don't need to go around removing Yonkos or whatever. They RULE the world. They aren't the challengers. They don't need to do shit because the status quo is in their favor.
So having....AHEM...a HANDFUL of people that can actually defy them directly serves the idea that it's very difficult to take them down unless more people band together. Remember that most coc users are actually really hard to work together. Rocks had that issue as his downfall, allegedly.
So, when you take a step back and look at the big picture, you kinda see that it's not much of a big deal as one could initially think.