But the KC-X main mission is for aerial refuelling
I hope not.
Main mission for KC-135 was and is just aerial refueling and that is the main problem for KC-135. KC-135 was used for less than flying 400 hours a year. Therefore lifetime expecting for the airframe is more than 80 years. But not all parts of an aircraft will last such a long time (skin, wiring, electronic updates...) . From an economic point of view this is a catastrophe.
I hope main mission for KC-X is going to be airlift so the aircraft can reach more than 800 flying hours a year. So USAF can retire the KC-X in 40 years with no regrets.