Another thing to consider.... How many KC-135s are there at the moment? 450 is it? Was this contract not for less than 200 frames? Does that not leave space for future procurement, or is the USAF going to reduce its tanker fleet by around 60%? And what of the National Guard et al?
This was the KC-X competition to replace the first 200 tankers in the USAF. There will be two further competitions to replace the remaining KC-135s and the KC-10s.
The reason for the 3 competitions was for several reasons. Firstly, they don't need to replace all the fleet now, so can afford to wait to take advantage of new technologies etc. Secondly, they don't need all the tankers to be of the same type; depending on the threat/need in the future they may buy smaller tactical tankers or larger strategic tankers - or both. Thirdly, it keeps the first winner on their toes for price and support since the level of satisfaction with them of the next few years will undoubtedly influence the next buy.
The KC-Y competition be around 2023. Expecting the 767 to be gone, and Boeing's own comment that the 787 is unsuitable for the role (don't ask me why), they are thought to be exploring offering a variant of the X-48 blended wing - lots of room for 2 booms and freight, and mil pax can't complain about no windows, roll rates etc. EADS/NG can offer more KC-45s.
The KC-Z competition is expected around 2033. I won't even guess the requirement or the possible candidates.