There is nothing that says you need to abandon because the Tank left these are highly popular dungeons since everyone at endgame is farming poetics right now. It usually takes a few minutes max to get a new Tank. I've never seen Tanks leave because of the party line up i did Expert roulette yesterday with two Ninjas in the party and nobody left. The only reason i normally see a Tank instantly leave is if somebody has low gear. You need to understand the rest of the party are people too and the DF is not a personal people dispenser for you to get a group to do things with. There is no contract you sign when you queue that says you need to complete the dungeon with that party of course there are penalties for abandoning the party and some would rather waste 30 minutes than over an hour of banging their head against a wall with a struggle party.