Note also that the last version of SimCity was launched in 2003, running on a completely different engine with a completely different simulation model.
The Maxis team has talked quite a bit about the challenges in making their agent-based simulation engine, Glassbox, performant on low- and medium-spec PCs.
The bit that stands out for me is that the last SimCity was single-threaded so even on modern multi-core machines it chugs.
SimCity is one of those games that would benefit hugely from multi-core computers so I find the whole "We moved core functions to our servers because your computers are too puny" reasoning to be lacking.
It seems obvious that the only reason they moved processing functionality to their servers is because they have to calculate the inter-region stuff which they cannot reliably farm out to untrusted clients.
Basically they decided that the new SimCity would be a "Social" game from the outset and architected around that, the DRM side-effect is just an added "bonus" for Maxis/EA.
Of course many people (Myself included) just want a new single-player SimCity... Hell I would even take the last SimCity with corrected routing and multi-thread support.
Indeed, many people are unhappy that this is not an upgraded SimCity of old. Perhaps that would have been a good thing, perhaps not, but that is certainly a different product than what they shipped. Either way, SimCity (2013) is an entirely different game from SimCity 4, even though the name is shared.
I'm not sure to what degree exactly the new SimCity was optimized for multicore, but just a cursory glance suggests that is making use of all 4 cores in my machine. In terms of what they actually shipped to be calculated in the cloud, my guess would be macro-economic and trade conditions, as well as regional dynamics. These require a view of all the data from the region, and are thus best computed live centrally.
The Maxis team has talked quite a bit about the challenges in making their agent-based simulation engine, Glassbox, performant on low- and medium-spec PCs.