This makes the assumption that if they make it, the market will obviously move to their product. The genre has a fairly fixed market, and taking away players from their previous investments (time and money) will take an exceptionally good game.
Ironically, making good, interesting games is the one thing that Activision/Blizzard are/have been shifting their focus away from, ever since the iconic "I want to take the fun out of making video games" statement from their CEO.