Hypothetical question: how would the balance of the game be skewed if the three tank stances didn't come with -20% damage?
So Shield Oath/Grit was literally less damage taken and extra threat, and defiance was threat, health, healing and access to different moves. Paladins and Dark Knights would stay in stance 100% and only drop stance when they wanted to tank swap and dps something without having huge threat. Warriors would probably sit in Defiance a lot more and mostly just stance dance to use specific moves.
I only ask as that was how i've seen other games do it - take WoW for instance, where the tank stance for protection paladins was literally "you do more threat with this on" and nothing else. Outside of PvP balance, tanks didn't really suffer a damage decrease in their tank stance (and if they did, it was compensated for by access to moves/talents that ended up doing more damage when tanking compared to a DPS stance).
The issue right now is that as Freyyy has stated, Warriors are designed to stance dance because they have a kit that allows access to different moves in different stances. Stance swapping is accessing different parts of the toolkit. A Fel Cleave is -far- more damage than an Inner beast both due to the potency AND the damage down of Defiance. Equilibrium has both uses. I guess steel cyclone/decimate is fairly similar. But in general, Warriors have been designed with the dancing in mind.
Paladins and Dark Knights -seem- like their stances are merely a "role designation" from the view of the devs - they're either in "I'm tanking something" or "I'm the OT and just DPSing at the moment" modes, as they're so much clunkier to toggle and they don't have anywhere near as much impact in the skill use. They're purely a "do I want 20% damage or 20% less damage taken" toggle that doesnt flow with their abilities, but the player base has minmaxed to the point where they're using a clunky kit in an optimal way that -probably- wasnt how the devs intended the class to be played.
I'm not saying removing the damage penalty is the answer, but it feels like if they arent going to make Grit/Oaths easier or more fluid to swap in combat, they should make tanking in the tanking stance more desirable. What if, for example, they hugely buffed the threat and damage of Shield Swipe and made it apply the Slashing Down Debuff.... but you could only use the skill in Shield Oath? Something like that so paladins and drks feel like actually being in their tanking stance isnt for the most part an entirely negative thing.
Most tanks now, on hard content, use their tank stances in the same way they might use a short duration cooldown: incoming damage, pop tank stance + rampart/skin or something, absorb big hit, back to DPS stance. This feels fine for Warrior due to the fluidity of the kit, but just feels "off" for the other two tanks.