The problem is if you envison the combat system as a lovely bowl of delicious stew. then what the devs have basically done over the years is take all the ingrediants out.

emnity may have been the potatoes,
TP the carrots,
MP the leaks,
Debuffs, the beans,
old ast cards the lentils,
cleric stance the onion.
etc etc. All added together made a fairly nice tasty stew.

what the combat sytem has become in recent XIV is nothing more than a bowl of water.. and if all they did was add emnity back then it becomes just a rather boring bowl of boiled potatoes and still lacking all the other ingrediants that made it tasty and flavoursome.

any one ingrediant on its own doesnt make or break a combat system it's how they all work together. and you can't make a delicous bowl of stew without a delicious mix of ingrediants.