It is indeed possible to make a MMO-ish game, depending on how you define "MMO" going solo: http://www.quelsolaar.com/love/index.html
I am sure it require a certain kind of person, But given enough time and patience and knowledge, the right person would be able to pull it off. Don't picture anything like World of Warcraft alone, though. It is still possible to make something simple in my opinion. Just have a plan and work slowly. Don't underestimate the amount of work. It will take years, and you won't make the new WoW.
EDIT: Here is an article about this, that shares my view on this: http://www.gamedev.net/page/resources/_/technical/multiplayer-and-network-programming/building-a-3d-mmo-using-websockets-r3392