Errr, nothing. "Stealth" isn't a panel you bolt on to an aircraft (despite the fact that you can coat one with *classified* materials that will reduce its radar returns. You need to change the shape to the point where radar is reflected in a direction as close to normal to the source as possible. Once you change the shape too much, you now have other issues that require more changes. A completely new design is far more practical than sprinkling magic "stealth" dust over an F-16. Ditto that with vector thrusting, etc.
Part of the problem is the incestuous relationship between the aircraft manufacturers and the DoD. Cost overruns are simply assumed (and paid), and everyone gets "comfortable" on the money spread around. A complete change in procurement procedures would be necessary reform in order to fix this. But then the people who get rich off of this (and donate lots of cash to politicians) would get hurt, and we can't have that... now can we?
Requirements “creep” by the DoD has killed many viable and initially, on target programs. This one is no different. They want new widgets and gizmos in the airframe, well, there’s a trade off. It gets heavier, thus slower. Then the subsequent trade off, performance.
Yet, the contractor is always the punching bag for doing what the “customer” asks and pays for.
No win situation.
When programs like the U2, SR and F117 were in production, they were either under or on budget because there weren’t thousands of acquisition people running around putting their “mark” on the airframe.