The game is designed almost entirely around casual players though.
Also PvP hasn't "ruined" PvE, they've got virtually nothing to do with each other these days. We hardly even see class balance and they balance the two separately now unlike in the past. The GCD change wasn't made strictly for PvP it was also made for PvE, probably the MDI specifically since that has been their big push.
The reason PvE is shit is because gearing is completely out of whack due to Mythic+, and outside of Mythic raiding and very high M+ keys there is no challenge left in anything because Blizzard is terrified of somebody not seeing something.