How about this?
When a player leaves a group in mythic dungeon, that player will receive locked (not depleted) key, which will require completing one m+ dungeon in time to become unlocked (usable) again. If the key used was the leavers, then the key will become also locked.
Then, Leader of the remaining group is given a 1 minute timer to decide whether or not they will want to reset the run (key will not convert into depleted).
Ez pz.