They've also said this is never going to happen. The game was designed from the start to be always online. Also apparently most of the simulation happens on their servers rather than your computer which is why it's also never going to happen.
ALL simulation is done on the client, EXCEPT for interactions with neighboring cities when they are run by human players. But in the single player mode even that is not an issue. The only thing the servers are needed for is persisting (saving) game state and that's the end of it. People have been able to play the game without internet connection for hours on end before their game crashed, clearly proving that the game DOES NOT rely on the cloud for simulation.
Besides, an average gaming computer with Core i5 and 4 gigs of RAM is far more powerful than some virtualized EC2 instance with limited and shared CPU/memory. Offloading computation/simulation to the cloud for "performance reasons" is a ridiculous notion.
This is about DRM and everyone knows it.
Other than that though, your explanation makes the most sense.
Saved games and inter-city communication is done against the game clock, not the wall clock, so they could reduce server load (on the only two things servers were doing, saving games and providing intercity communication when multiple people were on in the same region) by disabling the fastest speed of game time.
And you conclude from that that the game does not need the servers (explicit DRM or not)?
The game depends on the servers, either for DRM or for game saves. But when they are off-line, it doesn't matter the true reason for game crash.
The fact is - the game needs the servers being ON.