In 25M, losing a couple key players can actually be more like a college/university level class losing the Professor and some or all of the TAs. When a TA has to step up and fill the role of Professor AND all absent TAs, that's when it get's really dicey. Now you've got raiders who normally have a dedicated leader for their role in the raid reporting to someone else, who may or may not have the qualification to lead them. Incompetent until proven worthy, in this scenario. Raiders assume their new leader, a melee main for example, cannot possibly know the intricacies of being ranged, healing, or tanking, and many chose not to listen, consciously or subconsciously, which guarantees failure. This failure of cohesion then reflects poorly on the substitute, whether or not it was actually incompetence on the substitutes part that caused it.
Take it from the substitute: When the cats are away, don't try to be the new cat!