This is not new. If you are a 10m guild never settle for a roster of 10. Keep 11-13 players and rotate them in. Yes it sucks during progression when not everyone gets all first kills but it will pay off in spades towards the end of patch cycle when 1-2 people inevitably disappear. I've never raided a whole tier with the exact same team...someone will almost always leave. From what I've seen playing since BC is that this is part for the course.