It's happening in 7.2 anyway (in that the three current Mythic-only dungeons are getting Heroic queues), and the 7.2 dungeon is shipping with Normal and Heroic versions as well as Mythic, so that tells me the Mythic-only test wasn't as successful as Blizzard would have liked, despite name-checking the large number of Kara groups formed in 7.1. Which I think is a fine middle road; if anything I would close what difficulty gap there is between Heroic and Mythic, make level 110 Normals the current Heroic difficulty, and add extra side-bosses to Mythics as an incentive to keep running it on Mythic/Mythic+ when the queues come out.