There are a few reasons why it would be good to remove tank stances.

Implementation
Stances aren't very well implemented. They're treated as a buff which lasts until you disable it. The problem is that disabling a buff or "clicking it off" only can be activated at the end of your GCD. So while it's technically an off-global action, it often "sticks" if you try to deactivate it too early, giving you the false impression that you've removed it.

This problem happens any time that you swap between a stance and "no stance". It's a problem with Grit, it used to be a problem with pre-Stormblood Cleric Stance, and it used to be a problem with pre-Heavensward Defiance.

The solution, if you want to keep stances, is to always have jobs swap between two or more mutually exclusive stances. This way, you're never directly cancelling a stance. You're always activating a new stance which cancels out the old one. WAR and MNK have the best implementation of this, because they're always switching between two oGCD stances.

If you want your player to use your stances, then you can't punish them for swapping between them. Otherwise, you create an environment in which players are encouraged not to swap.

And this also brings us to the broader issue with tank stances: some jobs have it much worse than others. Some jobs have dps penalties for swapping into stances, either in terms of GCD or resource costs. Others don't. Some jobs have ways of negating the penalties associated with tank stance. Others don't. Removing stances is one of many ways in which you can level the playing field.

One stance to rule them all
The problem with stances is that there is one stance that is considered "always optimal". It's never about finding which stance is "situationally best". It's about maximising 'dps stance' and minimising 'tank stance'. So the solution generally involves figuring out a way to stay in one stance for the entire fight.

If you want to have "stance dancing", then not only do you need a system which gives you the flexibility to freely swap stances (see previous section), but you also need to give the players equally desirable stances to choose from. DPS benefits are out. The instant that one stance has a clear dps advantage, that becomes the preferred stance. But if you're comparing two unrelated benefits, it becomes less clear.

Let's take MNK as an example. Let's take Fists of Fire out of the equation. Let's say that Wind increases your movement speed but increases your knockback distance, and Earth increases your defense and reduces your knockback distance. Movement speed is generally preferable, but there are situations in which you might opt for the added defense or the knockback reduction.

The other way that you can make stances interesting is by having them modify actions based on the active stance. But this only works if there's no clear dps benefit of one stance over the other. This again comes down to trading off situationally beneficial effects that don't provide a clear dps advantage.

Stance-gated abilities
It's worth noting that this last point only applies if players can readily swap between stances. One of the big complaints about DRK this expansion was that a lot of our abilities were gated behind our choice of stance, despite the fact that changing stance is very expensive (both in terms of GCD costs and in terms of MP costs). This was compounded by the fact that DRK had a relative action deficit as a consequence of the role action system and the Stormblood action purge.

Stances can work, if implemented properly, but they definitely aren't suitable for every job.

Cleric Stance
One of the arguments for keeping tank stance is that it allows content difficulty to be adjusted for a variety of player skill levels. Not comfortable with the content? You can use more tank stance. More comfortable? Use less. So instead of the binary "survive" or "die", you end up with a spectrum of players with more or less dps output based on their stance usage.

Back in Heavensward, we had a similar debate over Cleric Stance usage. A lot of experienced healers argued in favour of keeping Cleric Stance as a means of differentiating player skill. And aside from the implementation issue that I discussed in the first section, Cleric Stance was probably a lot less problematic across the board than our tank stances are. There was no cost for swapping. No stance gated actions. It was the same for every healer.

But looking back at Stormblood, I can't say that I've seen many people miss the old Cleric Stance. There are always new challenges, and new ways of optimisation. The world did not fall apart. We moved on. The old system was there for the sake of being there. I suspect tank stances are much the same.

I think the bottom line is this: you can have stances on a tank, if it fits the job thematically. The key elements for doing this is to keep the stances oGCD and free, such that you're always swapping from one stance to another (rather than disabling a stance). You also need to ensure that one stance doesn't have a clear dps or mitigation advantage over the other, such that there are situational benefits to using every stance available. Hotbar space isn't really a big problem - you just need to set it such that pressing the one stance swaps out the hotbar icon so that you only need one slot for two stances (kind of like how Inner Beast/Fell Cleave work).

But I also don't think that stances need to be there on every tank, and in particular, I'd like to see PLD and DRK move away from this system. There are plenty of ways to do what we've been doing without having dedicated stance buttons. I don't need to press a button to declare to the world that I'm starting to tank.