I don't think they expire that quickly. It's true they would have been replaced eventually, but this is accelerating that schedule for some munitions by a decade or two.

That $900B/year figure is for peacetime (or what passes for peacetime in an empire constantly involved in small conflicts around the world) and peacetime rates of equipment/munitions replacement. This is the most intense air war the US has been in since at least Desert Storm, maybe Vietnam. That level of expenditure is not already paid for.