Why would it push DPS outside of parties, if it still leave tanks around 70% of a same geared DPS ? Since calculations would line tank DPS all the same, the stance could only be here for ensuring the threat management rôle.

While there are few games where tank are so "role locked", tank gameplay is usually fully tied to their tanking stance and rôle, and their performances are quite low when doing something else. Since healers are no longer "half locked" by Cleric Stance, it seems quite inequal that tanks remain locked into a lower damage output than they could.

While I do not favor the loss of gameplay, the fact people will allways search to minimize their resistance to the strict necessary avoiding one-shots, and crawl for any minimal DPS incrsease at the potential cost of their healer's sake (or putting the whole team in danger for pug content), it seems clear the FF design of stance that are not so required is flawed, or their content is.

Minimizing the damage penality would already lighten the feeling of punishment, like 10% less damage done for 20% less damage taken, as it would be a tradeoff which end favoring the whole efficiency.

The design of tank busters and enrages also favor little windows of tankyness with large windows of "just whack them all", and the STR trinkets were a consequence of theses design too.
More boss with high sustained damage could change the way around, but it seems out of their ideas for now.