Do we really need this aircraft? Almost certainly not. It may perform far ahead of any competition but by the time anyone else (China, India or perhaps Russia?) gets to this level we will be using UAVs for almost all dangerous mission anyway!
The US has been working on the JSF, and it's bigger and badder (and canceled) cousin the F-22 Raptor, since the 80's. I remember playing games in 1990 that featured them.
With a 30-year development timeframe, it's not terribly practical to wait until there's an actual threat to start building cutting edge military aircraft. So, it's understandable why planners would keep working on these things, as it's always sort of hard to predict what's going to be needed in 30 years.
A proper conventional forces treaty, if signed by all the major powers, kills this dynamic entirely.
Of course, Russia and China are not terribly likely to be interested in such a treaty, given how anything that locked the powers into the status quo would lock them into a permanent subordinate position.
