Presumable working on the assumption that the production line will be shut down after they are the last airframes to roll off it. Also a fast reducing commercial fleet in service at the same time it is expected in stay in service for around 30 years?
Achievable, after all kC-135s, AWACS and JSTARS have been successfully supported long after the equivalent civilian fleets are long gone - but obviously needs planning in advance.
Boeing starts work on replacing Air Force One
......"Additional modifications will be made to this contract in the future to purchase the commercial 747-8 aircraft; The Air Force said it wants to “own enough of the technical baseline to permit competition for modifications and sustainment throughout the aircraft’s planned 30-year life cycle.”...........
Boeing to Cut Production of 747s
Production already down to 0.5 a month, must be marginally productive, and with bursting order books for 777 etc the space could be much more profitably used.....