Well in wow, the raid I ran with ran the same 25 people every day. If we had someone sick or something we used someones friend or someone in the guild that never really ran and just dealt with it. The linkshell I was in was had people on the sidelines but the content was completely different, it was open world and you popped stuff with pop items. Some linkshells just managed with 18, some that couldn't do it with 18 had 30 people on the sidelines and they swapped out dead people mid fight.

I see what your saying about coil, but what else do you propose, take the locks off and make it like any other duty? I might suggest letting people keep doing it but instead locking them on gear. E.g. you can fight it to help out but can't loot on anything till you catch up to the point your at.