Most people who hop into the thread with "hurr durr it's mythic you're supposed to give it all otherwise mythic isn't for you" miss the point.
The point is "mythic raiding guilds" encompasses everyone from world top 10 to guilds ranked ~3000, and there's nothing in common between those guilds. Even when you compare a ~300 ranked guild is completely different from world top.
Heroic these days trips over within a reset, tops 2, even for these lowest of the low "mythic guilds" that are atm stuck at 3/10. If we tell all these people "mythic isn't for you, get out", they'll have content for 1-2 weeks after the patch, then unsub because nothing to do. Mythic chrono requires more than anything in hc, it was the same in the so called easy EN, mythic Nyth required more than anything in hc.
You can see how widely hc is pugged.
The problem is wow devs have to lob together "super hardcore skilled & farming day n' night people" together with "somewhat skilled semi-casual people" in one bag of mythic difficulty because they're together combined such a minority already. Majority of people in the community atm are either unskilled, or don't give a damn, don't want to be in guilds and commit, don't care to put effort or improve, and for all these people we have various levels of puggable content that doesn't demand from you to farm your ass off.
I used to raid through open raid at some point with people who had regular weekly event for normal (it was HM & BRF back then), you could obviously see that these people don't care about learning their class or perform well, but they were having fun, so yeah, things like normal has a reason to exist.
Problem with mythic is though you either tune it around world top 100 and let 2000 or something other guilds who want to try mythic bang their head against the wall somewhere, or you tune it more middle-of-the-pack and you get accused that it's too easy, faceroll, undertuned and world firsts cleared it in couple of days.
I don't think we need 5th difficulty really to differentiate the "really hardcore" from "ones from whom heroic was just a tad bit too easy", we're already in the territory of bloat and we're having weekly threads "which raid difficulty would you remove".