"The original creative vision for SimCity was to make a game where every action had an effect on other cities in your region," Fox wrote. "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."
Fox then addresses claims made by Maxis general manager Lucy Bradshaw that the game simply couldn't function offline. "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 [six and a half] months working to write and rewrite core parts of the game to get this to work."
To drive his point home, Fox explains that SimCity was designed from conception to periodically check in with the game's servers. This allows players' cities to communicate with one another (and transfer water, power, garbage and other utilities), as well as the entire SimCity universe. Removing this feature involves writing new code for the game to simulate this transfer of information and resources, which is why it's taken so long for Maxis to remove the online requirement.
Finally, Fox thanks SimCity players for their patience. "We know you want Offline play in SimCity and we are really happy that we are finally getting ready to deliver it to you."