I think everyone's ignoring an important point. The aviation industry is extremely intolerant of accidents; especially due to manufacturing and design defects. This is a good thing for the presidents safety. The president doesn't give a crapy about fuel economy or the latest carbon fiber designs like the rest of us. They need something with a proven track record of safety.
Plus it's probably easier to add to the 747 Missle defense and reuse the radiation hardened avionics