While this certainly can be the case at times, you can't just make a blanket statement like this. It simply isn't how it is every time. If everyone is executing their job properly except one guy who keeps eating aoe's, too slow to adjusting, etc, that's entirely his/her fault. You are correct to say that it has a snowball effect however, but it is due to the player(s) that keep messing up.
Everyone has their role in a group. If a person can't meet their role, they get replaced. That's how it is with anything in life, be it ingame or otherwise.