They have comprehensive anti missile and anti air counter measures.
They are completely electromagnetically hardened which doubles to both protect against electronic warfare, worst case scenarios like nuclear attacks, and to shield against spying on internal wireless comms.
They can fly for up to 15 hours without interruption at full fueling and full weight capacity.
On top of their long flight time they can refuel mid-air off USAF tankers if need be (fighter jet style).
They stock two separate galleys for food preparation with supplies for 2000+ meals as well as weeks worth of fresh water.
They have a full onboard pharmacy, diagnostics lab, and surgical center including a full size operating room and all necessary imaging technology. With all accompanying staff of course.
They don't require any support upon landing and supply their own retractable vehicle mounted stairway and baggage loaders which are stored in the aircraft. Strictly speaking all they require is a runway. They can deal with everything else on their own.
And that's before you even start to consider spaces for staff members, a dedicated seating area for the press pool that travels with the president, conference rooms, etc as well as bathrooms, showers, sleeping arrangements, etc for those on board.
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Switching away from the VC-25As to a much less capable/independent plane will massively widen the attack surface for targeting the president and will introduce so many avenues for espionage or assassination that simply haven't been possible in the past thanks to the VC-25As.