3. buildings are mostly the same -> millennium old buildings?
Likely not millennium old. There was a very interesting show (history or discovery channel) called Life After People, once we stop maintaining them, the majority our buildings would disappear surprisingly quickly. A couple of centuries.
That was an awesome show! I caught it on History channel, but they probably have shown it on discovery too.
That said, we could argue that the buildings of the 22nd Century last longer than modern ones.