The Boeing VC-25A ("Air Force One") is a heavily modified 747-200. The two current airframes (SAM 28000 and 29000) were put into duty in 1990 and are slated to be replaced in 2017, although I suspect that latter date is optimistic.
These two 747-200s are among the last remaining 747-200s in service. I suspect that they are the best-maintained 747-200s in the world and they have considerable sunk costs in their custom rigging, but even when the aircraft were put in service in 1990 their model had already been replaced by the 747-300 (1982) and 747-400 (1989).
Why hasn't the VC-25A been replaced to keep up with the times? A 747-400 airframe is lighter, faster, and bigger (let alone a 747-8). These would all seem to be suitable features for what is most probably the most important aircraft in the USAF fleet. Is there some attribute of the 747-200 that makes it particularly well-suited for this role despite its obsolescence in the market?