EQ came out in 1999 providing a 3D version of what were MUDs.
During WoW's development you had Star Wars, City of Heroes, Asheron's Call and Ultima Online.
It was an established niche market well before WoW came out.
Even with the release of WoW, Blizzard was taking a risk. Warcraft's fan-base was primarily RTS players, not MMO players.
Learn your gaming history.