The next generation of fighter aircraft are taking upon themselves some of the mystery of Dr Who. No one knows what the next incarnation will look like or what he/she will do; though there is a lot of speculation. Every generation takes longer to build, is more expensive and is undertaken with a finger in the wind: to see from which direction the opposition will come.
It is predicted that the fifth generation fighter aircraft will take fifteen years to reach a prototype. And it will be superseded in 2030. The USAF is wondering how they can plan now.
I could drone on, but no-one knows what comes next....This is an increasing problem for the slower timescales of defence procurement.