There's a lot of issues with how they did this expansion in terms of combat systems due to how they did the mitigate or die mentality.
Lets just lay this out with removing the defensive stats and just do some simple numbers.
Tank = 200 HP
Melee DPS = 100 HP
Ranged DPS = 90 HP
Healer / Caster DPS = 80 HP
Why does the tank have 200 HP? Because they got the highest HP and defense in the game, so if someone flattened it out they really have a massive health pool compared to anyone else, even beyond what is visible. So technically, to wipe the group you'd need to do something like 200 damage to everyone to make it happen, but the issue is how do the other jobs survive the attack if the idea is they want people to mitigate? The mitigations only do 5%-20% of the damage, so a 200 HP attack would only go down to 160 from the strongest mitigation possible, and mitigation stacking has diminishing returns.
And thus we arrive at the body checks. In order to make this whole design thing work, they would have to make it so that a 20% mitigation is necessary for people who are not tanks. However, healer and casters in this example are at 80 hp, so therefore at least two mits are needed for group content to survive as that means an AOE meant to wipe the group has to do at least 100 HP worth of damage. So then, to enforce the mit or die idea, the next mechanic after the AOE has to be something that requires everyone to be alive. Think the Limit Cut of p12s p1 or paradigma 3. Do you see where this is going?
And then they still have tank busters which have to deal enough damage to kill a tank without mits. This means that they got to either do 200 HP unmitigated to a tank, or have the first hit apply some kind of debuff to make the next hit do 200 HP or more to the tank. Well in this case they actually did both so the first attack will kill the tank unmitigated at 200HP, and then if they don't swap they will die because they got a debuff increasing the damage of the next attack by +100% or some crazy number.
This entire idea was stupid to begin with because outside of the hardest content available, your other content just isn't going to be able to enforce this mindset. People later on are going to out gear the content that came earlier to the point that they wont instantly die to anything except a tank buster. Thus, the aoe that would wipe non-tanks fails to do so, people ignore using a mit, then the coordination mechanic comes and everyone survives without ever knowing there was a check.