A Sims MMO would never work, EA even trying to make one shows the lack of understanding they have on why the Sims is the best selling franchise of all time.
A MMO needs to control the environment, all players must see the same environment and interact with rules and limits set by EA. The Sims is a sandbox life simulator that gives the player control over the environment and settings so that they can create stories and situations. The player can edit and create new lots, the more tools they are given, the more creative they can be (which is why Sims 2 OFB is so loved, it opened up so many possibilities). It is through these stories and situations that the player can set his own goals (have kids, grow lifeplants, organize a band, etc.).
A Sims MMO has to take away player freedom and creativity, and by doing so, a Sims MMO has to give the player goals to achieve and things to do. This is why Sims 4 seems so much more RPG and goal oriented than past sims games. Without EA set goals and achievements, there would be nothing to do. Sure you could go and talk to people online, but what's the fun of that? How's that different from instant messaging or texting? You could go "out" and role-play, but without the power to control the environment, it's less compelling. Furthermore, the addition of strangers and random people online make it even less likely that the player will be able to create the story or situation they want. Imagine having a nice dinner with your spouse when some naked blue haired guy comes over to hit on them or even try to fight them. You know this will happen. How can such a game be fun?
In short, the Sims isn't the type of game that can be made into a MMO. Both have features that work against one another and if you try to convert the Sims into a MMO, you'll have a game that will neither satisfy the MMO crowd, nor the sandbox crowd. You'll have Sims 4.
