I whole-heartedly agree with most everything said. It is just cheaper to base a tanker off of a proven commercial design. They do not need to be designed to land on dirt strips like the C-17, C-5, and C-130.
Interestingly enough, Boeing created an airliner based on designs for a replacement of the KC
-97 (Boeing 367-80) ... the Dash-80 developed for the USAF KC
-135 was turned into the Boeing 707.
I think the Air Force hit a home run with the purchase of the KC
-10. Not only is it a great tanker platform, it is also an excellent strategic airlift platform as well. It can move 27 pallets of cargo, weighing up to roughly 170,000lbs a distance up to 4,400 miles. In comparison, the C-17 has 18 pallet positions with a max cargo weight of roughly 170,000lbs but only a range of 2,400 miles!
I think we will see another tanker / airlifter combination as the next tanker for the air force. It just makes sense. Flexibility is the key to air power.
I don't have my in-flight guide with me, but there is a gee-wiz section of air show data to tell the patrons. It is something like this; it takes 8 KC
-135s, 2 C-5s, and 3 141's and 4 days to move a squadron of 12 F-15s, their support personnel, and cargo from the US to the Middle East. 6 KC
-10's can do the same mission, saving 1,000,000 lbs of fuel, and do it non-stop, truly giving the KC
-10 the nickname the Extender. If that isn't specialized, I don't know what is.