I did say "at any stage of mythic raid progression that matters". It's true that many AOTC people can do +20s towards the end of a major patch/expansion, boosted by the raid/vault gear that they already got, together with knowing the dungeon by heart from repetition and copying strategies from others, but that's not when it matters. Much less as the scaling will also go up in Dragonflight and even DF +15s will be significantly harder than SL +15s.
It's early to say, but by the looks of it, it's unlikely anyone will feel "forced" to do +20s per week to not be left behind, that will only happen in Hall of Fame guilds pretty much, and at that point, raiders love to get as good gear as possible by any means necessary challenging as they may be, it's part of the thrill of playing WoW all together.