Some DRK changes seem annoyingly simple yet are ignored....

[GoatOfWar]

I think the current issue with removing that functionality of TBN is you'd ultimately be removing the last real unique job mechanic of dark knight, in favor of making it a 25s Thrill of Battle with no flare to it. It would also be stripping DRK lot of flexibility in the use of mitigation as having Oblation and TBN on low cooldowns as separate buttons does has a lot of benefit to it, some people refuse to see it but it doesn't mean its not there.

Dark Knight needs more mechanics that reward it for taking damage, as I do view it was the revenge tank, so what you run into is that experienced dark knight players are already completely used to the TBN cost and its a borderline nonfactor, while to anyone who doesnt play the job hardcore view it was a major limiting factor. In actuality TBN does indeed give you a notable upside. It allows you to prevent overcapping MP without actually spending it, this is notable because you now have the ability to effectively enter a burst window with 13,000 MP playing into DRK's strength of stockpiling attacks for burst window that lead to its meta dominance of Endwalker.

I dont know about you but I'd rather have it as it is now than a 25s Thrill of Battle with 10% mit, and nothing else notable about it.

Being able to line up an extra Edge isn't really a bonus point for Dark knight when they streamline all the tank dps throughput irrespective of superior utility or sustain.
Rather, at that point it becomes jank. It's not rewarding, it's an extra step required to do the same as all the others.