Why would they? Don't get me wrong I love the game, but if other people were ready to move on when MoP came out, what has changed to bring them back? If you were already looking to leave this game nothing has changed to make you want to stay or come back.
The two games really offer completely different types of MMO experiences, so (at least in my opinion) they could easily co-exist without direct competition. Of course, this isn't what people on this website like to hear, so keep comparing the two and asking this same question (along with "is the game dying") every day until you feel like you have an answer.