I think in perhaps 8-10 years after SC2 has done its damage WC4 will come. Along with WoW, the RTS series of WC have been huge hits. There is no reason to think they wouldn't make another in the distant future.
I jumped from SC to WC3 like they were the same anyway. WC3 is an absolute epic game and its time has come to an end for many. Starcraft 2 is a welcome successor.