I am 100% certain that the vast majority of the 7+ million subscribers that Blizz lost since WOTLK stopped because Blizz ruined the server community with shitty features like cross realm and group finder, they made it too casual friendly and too easy (gearing, gold earning, leveling etc). to put it short, they ruined it.
So for Blizz to get more players back to the game they would have to do some drastic changes, changes that a big part of the current casual playerbase would not want.
So to make both groups happy, legacy servers are pretty much the only way to go, heck, i actually think that if they were to open legacy servers they would become so damn popular that blizz would eventually start changing the main version to how WoW once was.
Why would they not be able to focus just as much on the current version as they do now? If they were to do this (which i think they will) they obviously will hire more people, most likely the Nostalrius team, so nothing would change, or well, they would be making a hell of a lot more money.