The game doesn't track partial runs or instances created the way it would have to for your idea to go live. Blizzard would have to track an insane amount information to do that. Eyound the technical issues people shouldn't be prisoners to bad runs, slow runs or groups that misrepresents what they say they are doing. Things also do pop up and people just have to go sometimes. Than again I have never had someone take off mid group so I have to wonder about these people that keep having these issues.