The traditional justifications are:

1. The lack of war against our international rivals is in fact a huge success, indicating our powerful military is an effective deterrent. If we didn't have the F-35 we'd all be speaking Russian right now /s

2. Preparing for the last war is a perpetual problem for the military. That's how we ended up in Afghanistan with high-speed air superiority fighters, but no IED-proof vehicles. We should counter this known bias by buying stuff that would have been completely useless in our last major wars /s