They can't totally ignore the fact they have WoW and WC3.
There is too much lore in those games to simply throw them out the window. I doubt WC4 will be considered until Blizzard has noticed a huge drop in sales for SC2. For why would you want to compete against yourself?
Have you noticed the complete lack of love for wc3? they no longer support old patches of the game, haven't given WC3 any bonus units/heroes in many years. They care more for d2 than wc3.
When CONTENT is added to wc3 (not bugfixes/ladder reset) then it suggests wc4 is coming.
My guess would be ~5-7 years before they announce they are possibly thinking about working on it and a release date in 2022.