Those tankers operating now are 40-50 years old! I would rather have Boeings than some Airbus. The problem with trade ins is that they have a lot of hours on the air frames.
But the USAF puts very few hours on their airframes. Why pay for new aircraft when they will never reach their rated hours anyway? The highest time C-5s have less than 20,000 hours while a typical 747 built the same year accumulated over 80,000 hours. The first KC-135s aren't expected to reach their fatigue life till 2040. If the USAF buys a whole bunch of 767s at the end of their production, they will have the disadvantage of maintaining a 767 fleet long after the commercial 767s are no longer in service. Where will they get the parts to keep them flying? Will they have to spend a lot of money getting small batches of parts made? That's the problem they already have with the KC-135.