While there are certainly a large number of people who would be overjoyed at the release of WC4, it won't be a good financial tactic to release WC4
instead of a WoW expansion. The majority of the WoW community can barely handle pressing a few buttons in LFR, much less understand what's involved in a RTS game. With no expansion (and MoP for another year or two), the game would get run into the ground while all the RTS fans move to WC4 and the MMO fans just move to a different MMO.
I'm all for releasing both, but releasing WC4
instead of an expansion for WoW is just silliness