The underlying issue here is that tank stance comes at a dps cost, while other mitigation skills, excluding TBN and IB, do not. Now given that premise, if you have a surplus of mitigation, obviously the first thing you "want" to reduce is tank stance uptime, simple as that. They need to give us reasons to not want to reduce tank stance uptime, which basically means the healers must contribute more extra dps than what we gain from switching to dps stance, though in that case we're just shifting the burden to maximize dps to them, and they can complain over that as well.

Another possible reason to make us not want to drop tank stance is to make it impossible to survive in dps stance, but that has problems as well. As I said above, if there is a surplus of mitigation, you'd give up the ones that cost you more first, in this case the tank stance. So you need to make sure that the "free" mitigation tools like rampart, vengeance etc aren't enough to mitigate incoming damage, but tank stance is. Now if you make it absolutely necessary to use tank stance AND all those free mitigation tools (.e. no surplus of mitigation) to clear contents, you'd get people complaining that tanking is too hard. Let's face it, if the tanks in Angered, Rushers etc need to be in tank stance to survive, even considering their exceptional healers and party members, your average PF tanks won't live.

If you want to give some leeway for average players, there needs to be a surplus of mitigation, if you want tank stance to not be considered a surplus, you need to either make the free mitigations much weaker than tank stance (in which case they might as well not exist), or make them no longer free (i.e. every single mitigation skill comes at the cost of some resource/dps/uptime, and they must cost more than what tank stances cost). In the latter case they just become the new "tank stance" and people will avoid them outside progression.

Tbh considering how the devs want to reduce the skill gap between good players and average players, they might as well remove tank/dps stances from all tank jobs, just like how they removed cleric stance dancing from healers.