And, have you seen all the computers necessary to carry out a drone operation? I guarantee you not all of them are running an RTOS. Probably not even all of them onboard the drone.
Military acquisitions take a long time. To give one example, I know for a fact that there are airplanes flying right now that use DEC Alphas to control their weapons systems. Those planes first came into use in the early 2000's. An older version of that plane is still in use, and will be for several more years; you don't even want to know what it's using.
http://gcn.com/articles/1998/07/13/software-glitches-leave-n...
And, have you seen all the computers necessary to carry out a drone operation? I guarantee you not all of them are running an RTOS. Probably not even all of them onboard the drone.