I think going multiplayer just loses the essence of what the sims is. The players are the creators, we control every aspect of their lives. People can become very controlling about their sims and worlds. Sure, there are some random accidents and free will to let some control out of our hands if we so choose, but the game style doesn't lend itelf to being shared between players. Everyone makes their own stories and like some people already mentioned, it can be a way of getting away from it all, it's alone-time in a way, which some people value. No need to socialize and compromise for another player.
I mean, there are no points or competition, so for me, that entirely removes any incentive for multiplayer gaming. Shoot-ups, quests, races, those all work well in multiplayer, I can use skill to win or lose against someone else and actually, it's more fun than playing against a computer, but sims? What will interacting with another player add to the game? Your sims interact with each other (and they do it very well! :D ) you interact with your sims in a way, by controlling them, your sims might even let you know they don't like what you're doing by reacting in a bad way to what is happening in game!
As for making it an online game, I wouldn't like that. I'm generally not a fan of online-only games, because you can't really play them if your signal's bad or internet goes down. Sometimes, you just don't want to connect online at all. Remember, most people don't have their own, sole internet connection, they are shared with households, so I can imagine a lot of bandwidth being used for the game, then other household members using the connection for their browsing/gaming/streaming needs and before you know it, your connection's slowed down and the game become unplayable. Nope, with some very unfair "fair use" policies in place, it would put me off entirely.
