You're correct that they're used a decent bit in the defense industry. I can't talk too much about it for obvious reasons, but it's an area where I would expect an uptick in their usage within the next decade in the defense field alone. Check out the DoD's modernization efforts regarding seL4 and L4Re. In-depth knowledge of these microkernels is going to be a skillset in very high demand as defense contractors are forced to adhere to the modernization efforts pushed forth by the DoD.