The F-35 has been a disaster. It was meant to be introduced into NATO and other Western allied air forces years ago and it was also meant to decrease costs due to being a one size fits all type of design. The opposite has happened and has in fact ended up being more expensive then the now no longer manufactured and superior Air Superiority Fighter (and more stealthy) the F-22.
The RAND Corporation carried out simulation tests against Russian Su-35 (which is a 4th++ Gen air superiority fighter) where the supposedly 5th Gen F-35 was soundly beaten.
I think it's a case of where there has been so much money and faith put into the F-35 that to cancel it now would be a major political embarrassment, so many heads would roll, military aircraft procurement would be set back by at least a decade and US worldwide prestige would be badly damaged.
What's even worse is Russia will in a few years time start manufacturing their version of the F-22 and will make it available for export (India being the first to sign up- China making their own version). Being solely an air superiority fighter, it's likely to be able to defeat the F-35 in air to air combat so the US may have to restart their F-22 production and perhaps even lift the export ban on it.
