I'm not in anyway advocating the F-35 program be cancelled. It simply does NOT make sense to do so.
What I was suggesting is not buy it in the same numbers as originally planned and focus on the 6th Gen fighter. High-mid-low, not an all 5th-gen High-low mix of F-22s and F-35s.
Pretty much as localhost said, reducing the numbers of F-35's ordered would just drive up the individual unit cost, making the per aircraft price higher and causing other nations to reduce, cancel or not place their orders. This would in turn drive the F-35 price higher still since foreign orders are still significant in number.
The SDD is close to being completed, which means most of the developmental costs have been spent. Those costs need to be amortorized across the F-35 programme, which means significant reductions in numbers would only save the manufacturing costs for any aircraft not ordered. In terms of the total programme cost, that is not as significant an amount overall.
Also, attempting to develop a 6th gen aircraft prior to having the F-35 enter service is IMO premature. As the RAAF has found with the SHornets entering service, there is a significant jump in capability, attempt to 'leapfrog' developments can lead to information overload and reduced efficieny and effectiveness, since the capabilities cannot be managed or utilized effectively.
One of the other things which is key, is that in order for the 6th gen aircraft to be designed, a determination has to be made on what a 6th gen aircraft is... In basic terms, a 5th gen is 5th gen because of work done to provide it with the best possible control of situational awareness. Using Sig management, datalinks, advanced sensors and sensor fusion, a 5th gen fighter attempts to give the pilot the best possible 'picture' of the battlespace in a usable format, while at the same time attempting to reveal as little information about the fighter to potential hostiles present.
This can allow the pilot of a 5th gen to control the terms of an engagement, choosing when, where, how and who to engage/act on and forcing others to react. Until more is known about what capabilities would be required to allow a 6th gen aircraft such control of engagements, it is not really possible to start too much design work on them.
-Cheers