Think of the updates as damage control management.
The main project is wotlk,always was,always will be,even with the limited coin access in Marketplace.
Cataclysm is dead,they don't have the expertise to make mists work properly which is why they break as much as they fix every time they release a new row of attempted fixes,yet MoP actually does bring in cash which is why we see rows of ''updates'' once in a while - to give an initiative for new/old players,as soon as the numbers climb up - they stop,the numbers drop down - they resume.
Think of it this way: After siege of orgrimar has been fully ''released'' they will do the same thing they did with cataclysm,leave mists of pandaria dormant to it's own devices and release warlords of draenor beta realm even tho they said they never will,eventually as people migrate to warlords of draenor and other servers cataclysm will see an announcement of shut down,free transfers will be offered to MoP and other realms for those who stayed behind,and the circle will continue - cataclysm spot will be taken by mop,and what is now mop will be warlords of draenor.
Essentially they can always advertise them selves having the latest and greatest by having certain groups of people shuffled around newest expansions,while at the same time retain huge numbers of players by having a solid wotlk experience - and if everything hits the fan,they have ''best wotlk private server'' still going for them.
Inb4 the post gets nuked,a random dev cleans the topic,leaves OP comment and writes down what essentially is a PR copout :)