Shortly after release, Mark Jacobs released a State of the Game announcement. In it, he said that Mythic would spend more time and resources on improving RvR. This was when he announced RvR influence and a number of other RvR-centric features. He drew a line in the sand that day and said WAR was an RvR game first and foremost. By doing so, I believe he turned off many PvE players interested in WAR. The pendulum began to swing.
Apparently Land of the Dead is really appealing to the PvE crowd since its release, which is great, but there are many accounts that it is hurting RvR. This effect may only be temporary due to LotD's newness, but it just further highlights my point about the difficulty involved in balancing for different audiences in your MMO.
Does that mean they'll ever achieve WoW's numbers? Hell no. Accessibility alone gives WoW a huge advantage, not to mention it has now become a pop culture phenomenon.
That said, a more balanced WAR with strong PvE and RvR design could easily hold north of one million subscribers. How far north? *Ahem* I'm sure Mythic wouldn't mind feeling a bit chilly, especially after all the heat they've drawn since release.