I do agree that most generic vacuum tube hardware in the military is fairly scarce but deep inside some of the government administrative “hideaways” there still can be found some tube hardware gathering dust for that (hopefully not) eventual day.
On the other side (and to include Dan and Richard regarding tubes being alive and well) the military still uses and is also still requesting RFPs for hardware that may have tube-type items in it. Most of this is for equipment where solid-state cannot compete and is s usually found in the form of magnetrons, traveling wave tubes and other. Some of this hardware is specifically designed to provide extremely high output power levels to burn through the solid-state front ends of the adversary’s equipment in a non-EMP fashion.
The biggest problem with solid-state devices has been matching the power levels produced by their vacuum tube counterparts. To achieve matching power output a solid-state transmitter usually requires several banks of lower power modules requiring high total operating currents being combined to meet the requirement. And there are times when these modules may not like the load presented to them although manufacturers are now being very careful to provide necessary protection to avoid failure.
And solid-state is still vulnerable to nuclear events such as that found in space nuclear power plants. Obviously it would be nonsense to try to equip spacecraft with vacuum tubes. And development of things like gallium nitride devices for power levels and diamond field emission devices for both power and radiation requirements seems to be slowly solving these issues.
I don’t know if you are a member of the Association of Old Crows but the monthly magazine illustrates a lot of the cutting-edge military technology. My involvement has been based upon radar system development in the past from tubes to transistors and I still see tube applications being designed into new equipment.