You are absolutely correct, that tanking can be a thankless job. As player that plays all roles, when I tank low stuff, usually because of relic requirements, and I experience DPS as you describe. I have one response for them. "If you do not like it, kick me an wait another 30 minutes for my replacement." At this point they stfu and we continue or I get kicked, and can insta-queue to the next duty and a better group of humans.
There are rare circumstances, when I vote kick the abusive player.
Most often, I do not experience those fools. We wipe, meh, re-do, re-instruct on missed mechanics and try again.
Tanks that get emotionally injured by the bad attitudes of other player have forgotten just how valuable s/he is to the dungeon completion. Try doing any sub 50 dungeon above Toto-Rak without one.