Before we get around to building half the planned F35s, they'll cancel the program and move on to the next shiny new program. That's how every big defense project works.
Here's how it works. You start out with a giant estimate, for 3,000 planes. That giant estimate is mostly caused by the lifetime cost of maintaining 3k planes. As the years roll by, you start chopping production numbers off, while trying to get some cost inflation per unit somewhere. You end up spending 1/3 less, for 1/2 the planes in the end. 20 years after the first F35 flight, you've moved on to the next big thing. Conveniently justified by an inevitable, possibly dramatic, change in what the military needs.
Here's how it works. You start out with a giant estimate, for 3,000 planes. That giant estimate is mostly caused by the lifetime cost of maintaining 3k planes. As the years roll by, you start chopping production numbers off, while trying to get some cost inflation per unit somewhere. You end up spending 1/3 less, for 1/2 the planes in the end. 20 years after the first F35 flight, you've moved on to the next big thing. Conveniently justified by an inevitable, possibly dramatic, change in what the military needs.