[Accipitriform]

It doesn't necessarily follow from "the airframe isn't the most expensive part" that you still wouldn't want an improved airframe.

The F-16 isn't stealthy (terrible in fact), and the F-22 isn't well suited as a strike bomber - its weapon bays are too small.

In fact, the F-35's single engine design works well in providing enough space for weapon bays, while still keeping the aircraft fairly compact.