Blizzard hasn't produced even one single-player-focused game since... Diablo I? Their strength has always been multiplayer, I very much doubt they could deliver a blockbuster single-player open world game.
However I think that was the model of many of the older games. Once the game has been developed and released, single player improvements are largely either bugs, quality of life improvements or platform changes. The first 2 were needed less because of old school blizzard's dedication to quality.
Overwatch and HearthStone are much bigger jumps to be fair.