@cstross There are four things lurking down there somewhere.
Ferroconcrete has a max lifespan of about a century. Most office buildings are unsuitable long term assets.
The utility of the building assumes an energy price range for combustible energy inputs. We're leaving this range. (Cars AND furnaces.)
The utility of the building assumes it can be cooled.
Nobody priced in the first three.
As a result, whole lot of the economy is sitting on fictitious prices.