Back in MoP, we had our own, very well running 10man raid who was able to compete on our server (around rank 2-3 throughout MoP).
After Blizzard basically forced 20man upon us, we have been in nothing but trouble. We never managed to recruit the required players to continue playing on the same level, and it has been going downhill ever since. We couldn't keep up our old progression, became less and less interesting for players required to keep up with the progression we aimed for and that would result in players eventually leaving our Guild.
It is so incredibly hard to find 10-15 People that fit the raid, 10-15 people who play on the same "level" as the rest of the Raid does.
Now, in Legion, i have reached a point where i get anxiety to log in, click on the guild info log and see if another player left the guild because we can't keep up with what we used to be.
I will never understand why blizzard took away 10man as a viable format for Mythic Raiding.Sure it might be easier from a balancing point of view, but it still screwed so many guilds and takes away so much potential from guilds who simply don't have the options to recruit needed players.