My problem with 3e style AoO is not the existence of a free attack but the implementation and rules-bloat that comes with it. (And the implied reliance on a grid.)

You turn away from your foe? Free attack! That’s simple, intuitive, and quickly resolved.
(Plus: You do something clever to divert your foe? DM ruling, probably no free attack.)

About two inches.

It was a different time and people didn’t understand what they were playing.

Imagine Gary at the head of a table. Imagine 8 or 10 players looking at him. He asks for their actions. One of the players replies:

“I run up to him and cut both his legs then flip over the other person here and zap the troll with my wand of lighting, then drink my potion of flight and shoot darts at the rest of the people in the room!”

This is the third time this has happened this combat. Instead of taking the time to patiently explaining to new players that they simply must select one action, Gary takes the classic approach and just makes it happen for the player! This way they are not instructed by lecture, but experience. They know what they say they are doing is what they are doing. It also lets players know to think about what they are doing before they do it. It lets advanced players know they can use this to save time and eliminates the behavior from the new players.

The abstract combat system really doesn’t need anything else. (Maneuvers like grabs can, of course, be handled by ruling if desired, just like for weapon combat.) I’ve never really understood the tendency of D&D people to create baroque subsystems for unarmed combat.

I think you will find “disengage from combat attacks” in many old school games (they’re pretty common, and as you point out are a limited for of opportunity attack). I have already by habit used them once or twice in Pahvelorn, though I didn’t use any special terminology so it might not have been noticed.