The US Air Force does not want to get trapped with the F-35 when 2030 rolls around. Given that the partly stealthy light attack bomber won't be fully operational until 2019 that means they want its replacement or workaround coming on strength barely 11 years later. The link above will take you to a USAF study on what the US needs to maintain air superiority by 2030. It makes the case for a much better aircraft than what's on offer today.
Emerging integrated and networked air-to-air, surface-to-air, space and cyberspace threats, as well as aging and shrinking fleets of US weapon systems, threaten the Air Force’s ability to provide air superiority at the times and places required in the highly contested operational environments of 2030 and beyond.
Threat capabilities are likely to advance along two major vectors over the next 15 years. First, traditional threat systems will continue to evolve and proliferate. Along this threat vector are advanced fighter aircraft, sensors, and weapons. While near-peers have most of these capabilities today, advanced air and surface threats are spreading to other countries around the world. Air superiority forces will face growing numbers of these threats across a wide range of locations and scenarios in 2030.
The second threat vector is a series of comprehensive capabilities with a less predictable impact on warfare. These include increased threat capabilities to negate our advantages in the space domain, increased quantity and sophistication of cyberspace threats, and air threats including hypersonic weapons, low observable cruise missiles, and sophisticated conventional ballistic missile systems. How, when and where these capabilities emerge is less clear, but it is certain air superiority forces will face many of these threats by 2030.
The Air Force’s projected force structure in 2030 is not capable of fighting and winning against this array of potential adversary capabilities.
Then there's this veiled warning - let's never, not ever, repeat the F-35 fiasco.
No more "next generation" nonsense. No more building a plane at the same time you're struggling to develop the technology. From now on you develop technology through prototyping, the conventional approach. New technology is to be "harvested when mature," i.e. when you know it works and are sure it's worthwhile.