The idea behind 5 was more about about preventing the bots taking advantage of tuning knobs in enmity from the earlier points.
Like if the developers were so inclined, they already have a solution for this in PotD, you have to defeat X many mobs to unlock the portal to the next floor. That can also be the same strategy where killing specific groups of mobs opens different paths, or difficulty scaling, or different reward paths (reminiscent of V1.0.) Right now, you can wall-to-wall pull, right into the boss room of some dungeons. That is just broken. If a tank hits a mob, and runs away, it should lose interest after a few GCD ticks worth of time. It shouldn't be possible to just hit a mob once and keep enmity, but just the same, it shouldn't be possible to just run through the mobs and survive to begin with.
If a dungeon is laid out like so:
(Entrance)-(mob group A)-(mob group B)-(mob group C)-(First Boss room)
Hitting all the mobs from A, B and C, and making it to the boss room shouldn't ever happen, even unsynced. Throwing a door in the middle to require that group B be defeated is sufficient for opening the door, but another strategy could be to lure group A and B into a trap, and thus not have to defeat them at all. The door unlocks, and then you could pull group C into the trap,or just kill them directly. The only place in the game that really has something like this is MSQ's Castrum Meridianum, with the cannons (and was in fact the strategy before it was outgeared.)
But because players don't like obstructions to pulling everything, it makes the content slower. What would be necessary with luring mobs together or to a trap is some requirement that the tank or other party members not run away from them. I picked 15y because that's the WHM Medica distance, and is generally sufficient for mobs to consider a player “too far away” without a door resetting them. It can be farther away, it just can't be “so far away the mob can't hit the tank even once”