How often do you encounter that a healer has to leave before you get to the last boss, and after looking for a new healer for some time, everyone starts leaving the group. Now you're stuck at an unfinished instance. And that sucks.
There might be one way to make it work.
Just think about Violet Hold.
If you wipe, you have to start all over again, but you can't get any loot from the bosses you've already killed.
So, you killed 2 bosses in any random heroic, and the group falls apart. No way you can get to last boss.
But if you do the same as in Violet Hold, you can easily get into a new group, in which the other players that are new to that instance will get loot for the bosses you have already kiled, but you don't.
Problem solved.