One of the biggest problems with Tenacity is it's counterintuitive to tanks. Yes it reduces damage taken by a small amount but it also "Affects the amount HP restored."
The problem with this?
It only affects the heals the TANK DOES TO THEMSELVES, instead of increasing all healing done TO the tank.
Why this has never been resolved really does show how out of touch the developers are.
Determination affects healing done, Tenacity SHOULD affect healing received.
https://forum.square-enix.com/ffxiv/...ing%20you%20do.
1) That Strength meta resulted in a TON of extra stress for healers when subpar tanks tried to do it because it was the cool thing to do or tanks felt pressured to do it. A Strength tank was a lot squishier than a Vitality tank.
2) DH does help with emnity overall but emnity is largely a non issue anymore at relatively equal gear levels. Your example is only relevant when you have two tanks of the same job and the exact same gear other than melds. Even in that scenario, DH and DH-Crits are RNG so you don't know what the result would be. Also, the two tanks should never be in that situation. The off tank should hold off on tank stance for a few rotations and then turn it on and if there's not a tank swap should be Shirking emnity.