I'm a little opposed to this.

Sure, it's an inconvenience to other party members to have to wait for a new member, but seriously...

This also means that you are forcing people to potentially stay within a group that has zero chance of winning. Or 20% chance and making it incredibly stressful. Or even make someone run a dungeon they have no desire in, experience in, or flatout hate. That's not going to improve the mood, and the person might just flatout AFK or try and wipe the party out of spite to get them to quit.

Preventing people from leaving instances will never be a good thing if you look at the alternative outcomes of keeping them in...