I don't know how I missed this before, but this is interesting - Brad DeLong and Paul Krugman disucss an original post by Michael Mandel on the age of the capital stock. Residential and non-residential private capital stays about roughly the same age. Seems reasonable - it looks like market forces are maintaining an equilibrium that's probably pretty strongly dependent on productive technology.
Our public capital stock has not been at equilibrium - its been getting older and older. Its no surprise no equilibrium can be maintained. We know from the socialist calculation debate that non-market institutions don't respond to signals that would maintain equilibrium in the same way that the market would. The conclusion a lot of people take from that is "down with the public sector". Another conclusion oughta be "if we want the public sector to do certain things we need to manage it more deliberately - we can't expect natural forces to produce the ideal result."