Well, Cata heroics were kind of different since those were endgame for many as LFR didn't exists at given time. It was plainly way too hard content for some of the players. Meanwhile flying is only form of travel, and limiting it doesn't create any walls anywhere.
With WoD they can easily create harder heroic dungeons again, as there's another viable path; normal dungeons -> LFR, instead of having heroic dungeons between them.