...or why you can't handwave away endless grinds, RNG gearing and spec disparity as "it only matters for the top 0.1%".
There is no "I" in team. If you're a mythic raider, you are playing for the team first and for yourself second. That means if you have spare time to play, you feel compelled to do the thing that will help your guild progress further rather than the thing that you find "fun". Why do pet battles when you can grind more AP for stronger essences? Why collect transmog when you can grind M+ to get optimum corruption set up? Why play a worse spec when you can play a superior-performing spec?
Sure, you could technically succeed in mythic without the level of hyperoptimization explained above but let's be honest with ourselves. Is someone who plays for themselves alone going to scrub logs to correct every little mistake? Or be okay with being benched for progress? Or reroll when asked to? No, of course they won't.
There is no difference between the top 10% and the top 0.1% as far as their mindset goes. It's the "mythic raider" mindset. The willingness to pursue every improvement you can to help your team progress further, no matter how small. The difference comes from technical skill and that improves with years and years of play.
So the argument that "it matters only for the top 0.1%" is complete and utter BS because every mythic raider worth his/her salt is going to have the team first, me second mindset. And if you don't have that mindset, your guild won't make it far because the people that do have that mindset will eventually get fed up and quit for a better guild. Maybe minor improvements from optimizing corruptions and pushing neck levels don't really matter much for one particular boss. But when everyone in your guild has the correct mindset and keeps pursuing every little minor improvement, all of them together add up quickly and make for a strong raiding team. Endless AP grinds, RNG gearing and class / spec performance disparity affect all mythic raiders equally.