The reason we still have tank stance is due to the fact that they want to make content that requires more than one tank. It's the easiest way to handle enmity. Binary solution. Either you're getting the aggro modifier or you aren't. If tanks just generated enmity naturally, while we do have tools to make sure one is more consistently MT, it would lead to problems with tank swap mechanics.

It's really not a problem, and it's not too much to ask of you as a player. Before this you had to also pay attention to and use the skills that had enmity modifiers on them several times to build sufficient enmity. If you could do that, with this same "problem" still existing(whoever said it was taken out in Stormblood has misremembered), then you can surely handle this much easier system. If you want to completely delete the responsibility of tanks, then you might as well delete tank.

As others have said, it's simply following the programming rules of all job based buffs. They are wiped on sync. Just accept this small responsibility. It's the least they've ever asked of tanks.