The 777 is a tremendous upgrade. However, the 777 IS NOT what the USAF / DOD needs. We need a more tactical sized tanker rather than a strategic tanker.
As a tanker planner who was stationed at a fighter wing for a year, the problem isn't "gas in the box". There's plenty. The problem is, available booms/drogues and ramp space. With traditional aircraft, you can only put one boom per aircraft. And even so, it is faster and more reliable than two drogues. So if we replace our smaller and more tactical sized airplanes with big jumbos, where do you park them (ramp space) and how long will boom cycle times increase too since the fighters will be having to choose from less tankers in the air?
The 777 is a good replacement of the KC-10, but not the KC-135. But the KC-10 still has a lot of legs in her left.
In the future, I think we'll see the DOD buy a mixture of tankers. For example, a 737, 767, and a 777 sized aircraft (note, size comparison only ... not the actual airframes themselves).