You forgot to mention "and with everyone spamming m+", if you tune the raids as they were in the past, for "entry" gear, you create Emerald Nightmare that people considered "too easy" because they outgeared it very fast with m+ spam. You can't tune mythic around people progressing through heroic first, because the hardcore playerbase will have the same ilvl as full heroic raid set on end of week 2 when m+ stops being capped at normal raid ilvl.
And you can't do anything about it without fucking up m+ system, any attempt to put lockouts or limited loot per week onto it is gonna be met with endless rage. You have to count that by week 3 of Nyalotha everyone who cares about progression is gonna run in 460-465 ilvl gear.