Fedex decided to scrap 26 of the DC-10s they had in storage, rather than
mod them to the MD-10 configuration because the conversions were not
economical in the post 9/11 environment. Any airliner that has been in
service for 20-30 years is going to have serious corrosion and
maintanence issues. The USAF is much better off starting out with new
airframes, as they will probably have to serve 50-60 years, judging from
the KC-135.