I definitely remember seeing some DRKs trying to remove Grit after establishing hate though, in exactly the same sort of way some PLDs might have tried swapping to Sword Oath. It's just that you were far, far more likely to see WARs doing this, mainly because of the whole "Defiance doesn't reduce damage" thing (at least that was the reason I constantly remember seeing mentioned on the forums for why WARs hated being in Defiance), but I imagine not having to miss a GCD like PLD did was a pretty nice incentive too. That and maybe WAR was just most likely to attract the whole "DPS tank" mentality for some reason, idk.DRK's also cost a GCD IIRC but DRK's didnt and dont stance dance. they had both at all times.
Regarding DRK and Grit specifically, like I said I don't remember DRK back then too well, but it might also have been that activating Grit consumed a GCD but deactivating it did not, meaning that so long as you didn't have to reactivate Grit in the middle of combat, DRK wasn't losing any GCDs from it, unlike PLDs who had to actually swap stances and therefore use a GCD. I don't know if it was like that for DRK in Stormblood though; anyone who is more well versed in it can feel free to correct, because I'm quite possibly wrong.
Isn't that basically the core of what a tank has always been, though? You say that like it's a newfound problem with tanking in this game, but to me it basically reads off as "business as usual, then"Ultimately Tanking has lost any sense of actual tanking and all we are is just a weaker dps with more defense.
The only real difference is that in the past, tanks had to, in one way or another and to some degree or another, sacrifice some of their maximum damage output in order to actually effectively perform their role (whether that meant keeping aggro or not getting themselves killed). And let's face it; given a choice between having to do that or being able to do their maximum possible damage, it's not hard to guess what most people in this game would want to do. (If you were in my shoes during Stormblood in particular, it would not be hard to guess what most people in this game did anyway, even in cases when they shouldn't have.)