If they designed it more casual friendly you’d see mythic raid becoming more casual friendly, too, meaning you’d have more players attempting to do mythic raiding and participating in it so you’d have a bigger pool of players coming in to raid over what you have now.
This game is extremely casual friendly as it is. LFR, Normal and heroic raiding exists. Non-rated pvp exists. Solo shuffle, while it currently has problems, is a big deal for casual players. Mythic dungeons scale infinitely allowing players to play keystone levels they are comfortable with. Alt friendliness is at an all-time high.
Why would you want the most difficult content to be more casual friendly? They should just remove everything but normal dungeons and raids if that’s what you implying. Difficulty levels are inherent to gaming.