There's so much subjectivity there it's hard to even fathom where to begin. What does "no idea what they're doing" actually mean? You said you've done thousands of roulettes. What do you consider a "completely unacceptable" number of under-performing tanks? A thousand? A hundred? A dozen? Two?
You say you spoke to the person "politely," but in your own description of what you said here, you include things like "I'm having to tryhard to keep you alive," which I suspect most reasonable people would not consider a polite way of phrasing anything. There's also a lot of self-aggrandizement in the post and it just grips with condescension when you say things like "a level 90 savage raider like me." Get off your high horse already.
Frankly, it's completely reasonable to think mitigation gets mainly saved for bosses instead of trash, because A.) that's how it's frequently done in other MMO's, and B.) it's the bosses that have actual "tankbuster" abilities where Player Random can think that's where they absolutely need to make sure they have their mitigation ready, and the "death by a thousand cuts" from a trash pack just doesn't register the same way in people's minds.
Why do you have to take more responsibility? Because you chose to take on the role of healer. In no MMO ever has the "responsibility" been viewed equally among all party members. Tanks and Healers always bear the brunt of the responsibility. It's the reason you can play any MMO you want and those roles will always be in more need compared to DPS.
I was going to make what I felt was an awesome flow-chart to respond to this thread, but in the end only the last part really felt like it was needed:
-Are you making an enormous paragraphs-long post to rant about what you perceive as below-average players in casual content in a casual MMO, and then returning to the thread repeatedly to launch numerous other paragraph-long responses insisting you must absolutely be correct and anyone disagreeing with you must be wrong?
A.) If so, I think perhaps it's time to take a little break from the game. Taking a game this seriously can't be good for anyone's health.
B.) There is no B...