My understanding is that SCO was designed and balanced around player-player interaction between cities, to the point that a single free-standing city couldn't really be "complete." Cities functioned better when they were specialized and interacting with neighbors, than when they tried to be self-sufficient. I didn't play the thing myself, but that's how the devs seemed to portray things.

Given that (and other ways in which the online-ness was pervasive in game design), this seems like the epitome of a band-aid fix on a bullet wound. Just inserting an offline mode won't make the game better, because the game design was based around online play from a very fundamental level, and it's not a feature that can be removed simply. I.e., 'you can take SCO offline, but you can't take the online out of SCO.'