After the new expansion is announced at Blizzcon, it will be at least 6 months before it goes live. In the meantime, people are steadily completing SoO or just giving up and moving on. Factor in that the next generation of consoles and some amazing game titles are coming out over the holiday season, many people will jump ship and how many return depends on what comes out of Blizzcon.
However, given all this, how many subs do you think WoW will have left by the end of MoP? Given that as of last earning call WoW was at 7.7 million subs.
I think WoW will go below 6 million.