The big money for vanilla is going to be in getting people to buy the game upfront. After a few months, when everyone realizes they'll either a) never get to Naxx, b) have already cleared Naxx, or burnt out grinding the gear to clear it, or c) never made it past level 40 because leveling is a chore, the population will shrink to the hardcores. It's financially a lot smarter to get $40-60+ upfront instead