Sigh.. someone better start? People have been for quite awhile. Simply completing the missions on Legendary with the 5 debuff skulls active isn't really that difficult. At certain points in certain missions, it will be a tedious grind, but it is not out of reach for a large portion of HW players.
With that said, it definitely requires patience, which is what will prevent many players who are sufficiently skilled from ever doing it. The players who end up playing and beating missions like this are generally going for campaign high scores.
It's not exactly above high school algebra to quickly and easily understand how the campaign scoring system works. Now, your "mythic" difficulty is really only worth playing (in terms of mission score) on when the player can still approach a maximum combat/time bonus modifier, which we know is 10x/5x in Halo Wars. Allow me to explain fully..
HW Campaign score = (skull mod)(objective score) + (combat mod)(objective score) + (time mod)(objective score)
max skull mod = approx. 2.28x
max combat mod = 10x
max time mod = 5x
As you can see, the difference in turning all 5 debuff skulls is the change of the first coefficient from 1.0 to 2.28. This 1.18x change is often minimal compared to what most people could stand to improve from their average combat/time mods. See, what's a max score with skulls? 17.28*(objective score) Now, what's a max base score with no skulls? 16.0*(objective score) Unless you're already at or near that 10x on Combat and 5x on Time while playing Legendary, which you should be if you're going to handle it with skulls on as well, there's no point in really dealing with them. Personally, I'd like a heavier mathematical effect from turning on the skulls as they're pretty drastic all 5 together.
Now, as I've said before, there are a few missions simply/easy/quick enough where rolling Legendary with 5 skulls (excuse me, playing mythic..) is something the player can handle while still perfecting combat/time as well. If you look at the SPC Best Scores on Legendary, the M1, M5, M7, and M8 scores are all 17.28x scores. So, not only can some people play this "mythic" difficulty, but they can absolutely dominate it just the same as other difficulties. Clearly, when not worrying about your time bonus, simply completing the game like this isn't as hard as you guys seem to think it is.