If there were ever any type of story progression added to the game, I think it should only affect Sims in the "unplayed" category. I would not want the risk of unexpected game bugs or errors destroying the many, many households I've created and filled my worlds with. I had something like that happen with MC Command Center and it changed a lot of households that I didn't want touched.
Also, at this point, there are many rotational players invested in their games and comfortable with the fact that the game does not interfere, so the devs would have to be very careful with how they implement something like that. By making it only affect unplayed households, it should lessen the chance of something going wrong.
