There is always stock that has to be used up before they go bad and the pilots and machines need flying time no matter what. It's not as bad as it sounds and the rockets have their warheads removed.
At those altitudes the combination of a very high closing speed (air is thin) and the short range of the gun creates a real risk of flying into the target you're trying to hit.
The YAL-1 747 Airborne Laser had a megawatt class chemical oxygen-iodine laser. In an unclassified presentation I was at they claimed a hubcap-sized (30cm-ish?) spot diameter firing from Seattle to Wenatchee (~200 km) with the capability to melt thru an aluminum ICBM skin in just a few millisecond pulses. I think we could pop that balloon.
