A new blog has been released with very good information on how much work it took to get the upcoming offline mode to work.
The original creative vision for SimCity was to make a game where every action had an effect on other cities in your region. As such, we engineered the game to meet this vision, setting up the player’s PC (client) to communicate all of its information to the servers. That means that our entire architecture was written to support this, from the way that the simulation works to the way that you communicate across a region of cities. So yes, while someone was able to remove the “time check” shortly after launch, they were unable to perform key actions like communicating with other cities that they had created locally, or with the rest of their region(s), or even saving the current state of their cities.
My team did, however, see a path forward towards Offline, one that would maintain the integrity of the simulation. Lucy once said that Offline wouldn’t be possible “without a significant amount of engineering work”, and she’s right. By the time we’re finished we will have spent over 6 ½ months working to write and rewrite core parts of the game to get this to work. Even things that seem trivial, like the way that cities are saved and loaded, had to be completely reworked in order to make this feature function correctly.