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 suprisingly quickly. A couple of centuries.
The actual structures might decay rather quickly, but the traces would remain on the landscape for millennia. You only have to look out at the countryside to see the traces of Roman roads, mediaeval land use, motte and bailey castles, and other earthworks many centuries old.
If we ceased to exist overnight, our cities and roads would scar the surface of the earth for thousands of years.