As someone who is both a raid leader and an M+ tank who got KSM in all 4 Shadowlands seasons, I can safely say that....
KSM is literally a joke. I've been clearing 15's on the day of the patch, with my squad we can get KSM in 2-3 weeks if we don't slack. S4 KSM took me 4 weeks cause I slacked with my Tyrannicals on week 2, meaning I needed to wait till week 4 to get Tyrannical clears again. KSM is easily attainable by any stable organized 5-man group that uses voice comms and min-maxes ever so slightly (correct Covenant, spec, conduits, legendaries, legendary combo later on etc). Now, let's contrast this with my Heroic Raiding guild....
Where it took us forever to clear Heroic Sepulcher. We literally got Ahead of the Curve like 2 weeks before it was removed. Why? Attendance and performance issues. On a good night we'll have 15 sign ups. 5 of those people are sub-par when it comes to performance, not even having the skills/knowledge to farm 15s on week 1 like I do with my main squad. I'm forced to take them though, because scaling is fucked up and extra bodies in raids are always welcome (more CD's, more people to do mechanics etc). If I had my M+ squad x4, then Heroic would be cleared on 1st reset and we'd be starting Mythic progress by week 2. Alas, that is not the case. Out of 15 Heroic raiders who consistently show up and enjoy raiding, I can safely say ~8 of them are Mythic raiding material.
That being said, we did get 1 night in Season 4 where we had 18 sign ups. I held a vote, the vote passed, we pugged 2 people and walked into Mythic Fated Castle Nathria. We managed to kill 2 bosses and got the Hungering Destroyer to 2%. Sure, some carrying was done in terms of DPS, but in terms of tactics and mechanics my raiders were stellar.
Would you look at that. The same group that easily farms 15s on week 1 so as to not fall behind on GV progress managed to muster 20 people ONCE (with 2 PUGs) in ~6 months of raiding. When we did muster those numbers, we were successful. But gathering those numbers was always the hard part, and always will be. To compare organized Mythic raiding to +15's is a farce. If you wanna compare Mythic raids to some M+ level, it's easily 20's+, maybe even 22s-25s.
Also, people are bound to ask, so here's my Armory link. Knock yourselves out.
https://worldofwarcraft.com/en-gb/ch...rhood/dalinos/