> a low-flying HCM traveling at Mach 6 may give a target only 15 seconds to react after detection
This is why I say, bring back the Naval 16 inch guns and instead of shooting shells, either have them shoot airburst or pellets. Those old Battleships had 6 16 inch guns that could shoot 1,200 pound shell, 26 miles. Technically, 2 battleships could cover an entire fleet with ammo that is way cheaper than the projectiles and drones that are coming in at it, by an order of magnitude. Guns could be prepositioned to cover the sky in at various altitudes, and ammo only needs to be strong enough to cause a detonation if not destroy a drone. As for air defense, solar powered blimps that can stay up for days armed with lasers, could give line of sight to any drones or missiles as another tactic. Flexible solar panels are light and getting much closer to the capacity of hard panels and would make an excellent out shell to capture sunlight at all angles from sunrise to sunset, without the need to be repositioned, but zoned. The future, due to missile speeds, are going to be carriers at safe distances that need to be constantly mobile along with the fleet.