I don't see it as an issue. Mythic dungeons are not entirely inaccessible to the common more casual player. I consider myself more casual in the spectrum but I started running mythic dungeons in WoD pretty early on after their release. I just needed to spend more time gearing up and I primarily ran with friends and guildies. The worst the mythic-only dungeons are going to do is force you to communicate with people. The gall!