Most of the arguments against subdividing the tanks seem to be based off of assumptions on what features these subroles will bring. Will MTs be unable to do competitive dps? Will OTs be unable to withstand damage?

There are plenty of ways to design this system poorly. Whether this will prove true for Shadowbringers remains to be seen. But I can give you two very, very good examples of how the present mindset of "let all tanks be able to handle every situation" leads to major tank balance problems: they're called "Heavensward" and "Stormblood".

Homogenising the tanks so that they are able to universally handle every situation only works if they are well and truly identical. But they're not. Even looking at the role action system as implemented in Stormblood, we see a broken system which preferentially favoured certain jobs at the expense of others: "What's yours is mine, and what's mine is mine."

The phrases MT and OT tend to rankle; there's a connotation that an off-tank is just a mere understudy (which is why most tend to favour the more equitable "co-tank"). In practice, the "OT" role in a fight (however you define it for that particular fight) can be quite technically demanding. There are plenty of tiers in which it's actually more fun to play Luigi than Mario (Final Coil being one of the most memorable examples of this). But generally speaking, once players devise a system of tackling a given fight, there are two set tanking responsibilities, with 1P covering one set, and 2P covering another.

I've covered this elsewhere, but this is a game about two tanks. And their names aren't MT and OT. Their names are PLD and WAR. DRK exists in a nebulous area between the two.

In Heavensward, PLD suffered because DRK was the better "PLD". You had slightly worse defensive cooldowns on the whole, but your reward was a dps advantage. There was the odd fight where PLD had the progression advantage (A7S being the most obvious example), but even if the fight was physical, you generally chose the option that gave you more dps. In Stormblood, DRK isn't a better "PLD" than PLD, and it's certainly never been a better "WAR" than WAR. So DRK participation rates fall.

The problem with the current setup is that, even if we add more tanks, they're all going to fall into the space between PLD and WAR, as more mediocre versions of the original two tanks. So to force a change, we need to break down the privileged positions that these two jobs hold. Two slots. Two sets of jobs competing for each. You're allowed to hit hard on a job that isn't WAR, because that's no longer a privileged spot. You're allowed to support your team on a job that isn't PLD, because that's no longer a privileged spot.

It's a bit like when Warcraft shifted from having Warrior as the defacto tank to having a bunch of jobs being able to be equally competitive. Except that we started with two subtypes of defacto tanks, instead of just one.

The other thing that appeals here is that it sets boundaries. The Shake it off debacle in 4.2 would have never have happened if there were clear limits on what each tank could and couldn't do. You can still feel like every job has some features that others don't have, without having complete exclusivity on an important ability that makes you irreplaceable.