Retire the C-5 fleet as they are old, have minimal operational readiness rate and consume huge budget keeping them.
The C-5Bs are being modernized with glass cockpits, new avionics, and new more fuel efficient engines. When complete, they will be C-5Ms.
We need the C-5's cargo capacity for lifting things that the C-17 can't.
There was a time not so long ago when this same argument went on regarding the C-130. "We have all the C-130s we want or need" the Air Force would say. Senator Sam Nunn and Congressman Newt Gingrich, both from the state of Georga (home of the C-130 Lockheed assembly plant,) would put a couple of C-130s into the military budget each year, keeping both the assembly line and the C-130 program alive.
Now we have the C-130J, the Air Force are buying them in droves to replace their older, tired Hercs, and we as taxpayers didn't have to go through a great big decade long drawn out competiton to design a brand new replacement for the C-130 that would end up costing ten times as much as the current aircraft.
The C-17 program also needs to be kept alive with a trickle of purchases. If Boeing gets foreign orders, then we don't need to buy any. If they don't, then we can order a couple each year as we retire the older C-5As that aren't being modernized.