The primary driver for requiring the STOVL variant was the US Marine Corps which wanted to fly the F-35B off of amphibious ships to provide close air support for ground troops. There was never any possibility of equipping those ships with catapults and arresting gear; not enough space. Now perhaps the STOVL variant should have been dropped and the Marines told to make do with other aircraft, but that wasn't politically feasible at the time.

Sixth generation tactical aircraft will probably be optionally manned. So they can operate as drones when flying high-risk strike missions against known targets but can also carry pilots when greater flexibility is needed.