...There are skills for avoiding opportunity attacks and flee immediate danger.
There's two of them, and one of them is useless because in any situation when you would use it, you're very unlikely to be able to because of the high fatigue cost. The other one requires you move someone else in harms way, so if you're planning to flee the whole field of battle, you're just exchanging one life for another.
I find the realism argument meaningless here. For gameplay, it would be interesting to limit AOOs to one, but only if all the other skills were rebalanced. Spearwall wouldn't make any sense as is, and a lot of the skills like rotation/footwork would become 80% less useful as well.
Fair enough, spearwall would obviously be excluded from this rule. You could also have a perk with a large fatigue cost that lets to enter a state, like spearwall or riposte, that gives you a whole lot of attacks against anyone who moves through you space.
A nice benefit of this is it would make acquiring armor easier, as there would be less need to fully encircle fleeing enemies before they get annihilated by four opportunity attacks from one person.