You mean about 30 people carrying their weight an 10 afk or herp derping? 40 man raids were nothing short of a clusterfuck from the very beginning. They'll never be back and thank god. I couldn't imagine trying to find 40 people who weren't neck beards at this point with the games community.
Also the whole pruning argument literally makes no sense. Scaling content/mechanics back in Vanilla was not nearly as complex as it is now. That's why 10man fights in some cases were often harder than the 25 man equivalent because of raid comp and fight mechanics.
Vanilla NAXX for example, you needed 8-9 tanks for 4 horseman, do you know how terrible that was? No thank you.