They will do whatever gets them the most money, but there is not much to do with WOW today. They have been working on it for years and they have done a pretty good job, despite the horrible graphics.
An expansion pack is not a good idea for them because making ppl who are adicted to the game buy another one isnt nice.. but it is a good way to make a profit. Ive never played WOW before, but i doubt it costs money to subscribe, so they arent making much off of it.