Adding an additional harsh penalty would also act as a double undeserved punishment for someone who would have an unwanted long (more than 5 min) disconnection from the game (because if such penalty would exist, you would have to penalize disconnected people as well, else people would force a disconnection on purpose to work around the penalty).

Not only that but a player who wants to leave a dungeon without getting a penalty, could then simply go AFK or provoke wipes just so other players would votekick him out, or quit themselves.

So it would not solve anything. I think 30 minutes is already a penalty enough anyway.
The real issue is the imbalance in population between tank/dps/healer. That's what need to be fixed.