I'm sure the expansion and amount of content that can be cleared every week plays a part because of burn out. The problem with turnover is that if it hits you all of a sudden it can be crippling to a guild. This is why a ~25 man roster is necessary. You need to be able to rotate players so that IRL issues and emergencies don't cost you raid nights. You start missing raid nights and the attendance issues begin to snowball and you start losing players. To small guild/group without a good bench that's devastating.