It would only work out in a binary way. If healers gain more damage than tanks loose because of tank stance, then tanks are now REQUIRED to tank stance and healers are REQUIRED to DPS more. Or it works out that tanks gain more damage than healers loose by taking off tank stance and we have what we have now. DPS tanks all day ery day.

As long as the stance exists and the stance changes tank damage output in anyway, we will end up in this either/or scenario.

If tank stance DOESNT affect damage in anyway then it becomes a boring toggle with no interaction. "Oh im taking damage. Tank stance should be on." About as 'interesting' as darkside is now. "Oh im hitting things, I should turn on my damage buff that unlocks all my good skills." Things like this can just be passive traits as they serve no purpose.

The oft suggested make bosses hit so damn hard you need tank stance or you die is just the other side of the same boring coin. "Im getting hit guess ill put on the tank stance or I die" isn't a choice. It isn't 'fun'. Its just another bland toggle you have to hit every time you tank swap.

As long as tank stance affects damage it will be min maxed with healers. Removing any penalties (or 'forcing' tank stance via cray damage) will make them entirely unengaging toggles akin to darkside with no gameplay interaction. If you do the latter you might as well just bake it in passively to tanks all together.

If you want engaging tank stances you cant just make shit hit so hard you have to use it. Engaging tank stances require incentives to both have it on and off and force interesting choices. That is what we have now, just the incentives for keeping it on are not as strong as reasons to take it off (enmity is derp easy for one). Forcing stances isn't enhanced fun gameplay.