This i'm wondering too.
My guild has been through the same thing, except it didn't involve mythic+. My guild, also 10 years old, is a social and leveling guild on our realm, and we have a little raid group.
We've been a group of social guildies that just wanted to play normal raids and do our thing, and at some point in EN, we done all bosses and cleared the whole raid in 1,5 hours. We had to move to heroic to keep the people entertained, but half wasn't "good enough" to go there. The top end didn't like carrying people, and after long discussions, they left the guild.
We're still doing our normal thing in NH now, but the day they left was a sad day
We've always done normal raids, for fun, together. Why not now :/