Animal are territorial so what's wrong with this whole "leash" mechanic? Why would an animal want follow you to an unknown area or perhaps one where they are threatened by something that preys on them, that's what's truly illogical lol. The monster stops chasing you cause it no longer sees a need to keep tracking you down since they no longer consider you a threat of invading their personal space. Not all mobs are like that though some will follow you for quite some times and you may find yourself running into more threatening enemies but overall there is nothing wrong with the mechanic.
I do apologize if this has been mentioned before but just because some of the more noticeable mechanics are gone doesn't mean they aren't still there. On the journey to obtaining the Exploration achievement you start to learn that there are still quite a bit of kinks to each mob, sure they look overwhelming at a lv 80+ level but if you proceed in a proper manner you have nothing to fear.
Some basic ones are the red crabs north of Camp Bald Knoll won't aggro you as long as you walk behind them and don't move when they are looking at it. Think of it as red light green light, you move while they aren't looking at you, then as they turn around and move you just stand as still as possible til they pass you. Though not all crabs act in a similar manner the green crabs easter of bloodshore won't aggro you as long as you walk past them regardless of if they see you are not. As for the drakes from what I've seen none of them will aggro you as long as you walk past them, the same goes for Efts. As for raptors they seem to aggro you if they see you, you might be able to run behind them, but even if you are standing still if they turn and see you they will aggro. There are some boars that you can walk behind as long as they don't see you though some boars like the Pugot are more aggressive and seem to aggro you if you get to close to them. Then the Morbol aggro if you get to close as but they are easy to outrun so if you can find a safe spot to hide at you can outrun them.
Overall the AI is still there, you just have to explore and find it for yourself, the more noticeable ones aren't there anymore but the aggro is quite varied once you start to explore and obtain each gate. Odds are other means of aggro will be added to the game later on but for now take a step back and appreciate that there are indeed some areas in which the AI is quite interesting to deal with.