I've never heard anyone, from 5e or any other system, with anything but positive things to say about Advantage/Disadvantage. So this surprises me.
Sorry this is a bit off topic;
If you took, the three action economy of PF2e, Proficiency bonus, and Advantage/Disadvantage. I think you can make an incredible simple, and deep, ttrpg.
If I was WotC I'd make 6th edition based on just those and swapping attributes to just the modifier. The math, assuming bounded accuracy, writes itself. It provides 3 tiers of effectiveness, PB, Adv, Adv+PB. It does everything you need out of the math.
The three action economy lets you drop weird rules like the restrictions on a second spell in a turn. Assuming WotC follows their own math, the home-brewing for this system is dead simple. Disadvantage on subsequent attacks is a cleaner solution then PF2e's current multiple attack penalty, you could also just deny PB on those attacks depending on how you balance it.
Adding complexity is easy with the 10 over or 10 under system on skills and statblocks. Kobold press is already using it in their new Monster Vault. On these items it doesnt have the memorization requirement of attaching it to various skill checks.
It's so clean. The basic rules could fit on 1 page. If WotC decides to do 6e without inventing anything new, I think this system writes itself.
It’s static though - you can’t have double advantage or double disadvantage. So there’s a finite limit to teamwork. It’s simple to comprehend and elegant but doesn’t have complexity. Eg: a rogue can now grant themselves advantage at will as long as they don’t move. What purpose teamwork and tactics there absent DM pressure?
Somewhere there’s a middle ground between 5e’s binary simplicity & 4e’s pile of modifiers.