Pontius,
You make an interesting point. However, the so-called 707 Trainer Cargo Aircraft for the NAEW&C fleet were actually under-used for pilot training both basic and continuity. They were used very much more for cargo and the transport of personnel, where it had great utility. Aircrew did not favour it as a trainer because it differed in cockpit configuration and performance to the E3. I believe its diminished use as a trainer and the availability of commercial logistics support led to its demise. Also, as a small fleet, of old aircraft, they had an increasingly expensive upkeep bill, similar per hour to the E3 itself.