I had the idea where the entity limit could have a priority list for certain mobs. For example, you could type /entcount priority villager 1 to make villagers only despawn if there isn't any other mobs. y=You could type /entcount priority mushroomcow 2 to make mushroom cows only despawn if there aren't any mobs except whatever is in slot 1. Or, you could put /entcount priority list to see the list of your priority despawns. It needs some work, but....
it could just be a server side preference really imo villagers should despawn last of almost every passive mob but who knows maybe this is in the works