It's baffling how so many people point to BC and Wotlk sub numbers and think that explains which kind of content/expansion most people should consider to be the best for the game. The end of BC and most of Wotlk took place in a time that was at the beginning of the Great Recession, which began the overall decline in the video game market. There is less interest in the buying and playing of video games today than there was back then. For example, Nintendo, with its host of iconic franchises and high level of innovation, has been struggling mightily. Any notion that WoW expansions following BC and Wotlk should have attracted the same amount or more people, simply because more time has passed, is false.
There are some companies that still are able to pull off very good profits, despite the economy and sliding video game sales. Blizzard (and Activision Blizzard in general), despite the nonstop criticism it gets from video game forum posters (i.e., people who are generally a lot more hardcore about video games than your average player), is actually pretty good at this based on the numbers we've been given so far for MOP and its other franchises.