DRKs have been asking for their old finisher for years.
Because it looks cooler and could be used as a way to give DRK more than just the 1-2-3 combo and an alternate spender after 2 in every 5 combos.
Not for an enmity combo that'd be used only a few times per fight (before essentially hot-swapping that combo off one's bars as not to have said bars bloated by it).
The issue is that then people might end up easily competing for Aggro (not that it doesn't happen nowadays...), and people will hard-focus DPS instead of grabbing aggro.
Most will grab aggro, because it's higher rDPS to do so than to chance the boss nuking a DPS during their opener. That's not the issue so much as that "fighting for aggro" after the first moments of a fight or add spawn is a pretty damn boring maintenance mechanic given how shallowly it's ever manipulated in this game and is very expensive to support in any way XIV would think to attempt.
But honestly, while not perfect, I did like the system they had in Stormblood. Especially with stuff like Diversion, which forced the DPS to also have some responsibility when it came to group content. But given the way the game's designed now and their philosophy, I understand why it's no longer the case. Seems to me they want a pick-up-and-play style rather than having to rely on team effort.
Diversion was removed because it was bloat, in much the same fashion as Invigorate. You just hit in on CD for a subtle rDPS gain if any only if you will be 2nd highest threat anyways
and all other DPS are likewise mitigating their enmity, too. Which badly conflicted with the way enmity window maximization worked for tanks, since they'd want to estimate likely enmity totals over the fight right from the start, and they kind of had to (erroneously at times) assume their Damage-Dealers weren't idiots.