They could have just made a Diablo or StarCraft universe MMO (or both) and people would lose their minds and sell their kidneys to pay for a monthly subscription...
It's really not that simple. An analogue in software would be "Ford could have just made a fully self-driving car."
MMOs are both technically hard and artistically hard, and even Blizzard has failed at making them in the past:
https://en.wikipedia.org/wiki/Titan_(Blizzard_Entertainment_...