I'd never want other people to be able to meddle with my gameplay/stories and take away some of the control. They can create a multiplayer game but they won't get any of my money for it. I think part of the appeal of the sims is how every individual has full control in their own game. No one can walk in and screw things up. This way it offers a form of escaping away from the real world. I don't think sales for a multiplayer version would be as high as the current numbers, because it takes away a lot of the appeal it now has with the players. Of course, it might draw in new players but it would probably lose a lot more than it can gain.
I do like the original and creative ideas in the starting post of this thread, but it's just not for me I'm afraid. I don't really see how it would work with families. Someone can come knocking and steal away a spouse or...?