How would it makes tanks different? You either need it, or not, doesn't matter what tank you are, you use it at the same time, you haven't changed anything.
You could make the fights more RNG, but what happens when you do the fight the first time and you have to spend a lot of time in tank stance and the second time you don't need to because the damage never came out frequently enough?
But, the big question is WHY? Why is it necessary? I could, again, point out everything, but why does a tank have to sacrifice their damage to put up mitigation? It isn't going to change anything anywhere near as much as you seem to expect.
As for the comment about 'losing 80 potency', my response would be, what is the point? If you have something that is so inconsequential that it doesn't even matter, why have it in the first place? It is there for no reason other than some misguided sense of 'role flavour' that serves no purpose other than, I'm losing a small amount of potency that someone arbitrarily added for no reason.
Also:
People still learning how the game plays isn't a good metric for gauging how effective a mechanic is. If we were to go back to the ARR days, but still had out knowledge and experience now, can you honestly say people would still be in tank stance when tanking, or, would they be in DPS stance. I would bet, with 99% confidence, that they would be in DPS stance. HW was the main turning point for most of the community with SB being the full switch over. Also, Warrior only used it occasionally when Unchained was available to negate any downsides to using tank stance. That should tell you all you need to know. Also, if you are calling the tank stances 'undertuned' (by which I assume you mean the defensive benefit is too small but the offensive hit is fine) then you are talking about a potency loss of more than 80, it is going to be quite a significant hit and this is before talks of whether it is an oGCD, what defensive benefits it provides etc. Which brings up the point, we have both been living in hypothetical situations where I am assuming something similar to what we used to have, however, you seem to have a different idea. Maybe it would be beneficial if you actually provided an example or 2 of what you expect with a defensive stance, a solid foundation to base a discussion on is better than talking from 2 different buildings after all.So, that "never" is... half the time tank stances were in the game, despite tank stance's gains being pathetically undertuned relative to its costs and a largely unusable mechanic on all but WAR due to its costs in changing stance