The way it trended in WoW was that the subscription numbers always drop down a bit near the end of an expansion due to content being exhausted and an update drought of 6+ months. But their overall subs kept increasing and stayed fairly high up for both their 1st and 2nd expansion. In the case of MoP though, I recall there being a signifcant subscription drop near the end (though this is still higher numbers than what they'd have during classic I believe), due to (and I can attest to this) the overall lack of late game content, especially for the release of their last raid instance. It's just not enough to fill in that incredibly long period toward the next expansion. The real challenge is getting those sub numbers up and retaining them for the expansion release.