If the F-35B had failed

What would the Royal Navy and US Marine Corps do if the F-35B were not approved or did not work?

They would extend Harrier service, while Britain would probably have to convert the Queen Elizabeth-class carriers for conventional operations.

The immediate response would be to extend and continue upgrading AV-8 and Harrier fleets rather than retire them. Britain would face the expensive and politically embarrassing need to convert its new carriers for conventional aircraft. The speakers nevertheless distinguish underperformance from total technical failure, arguing that an aircraft reaching final production would probably retain at least basic takeoff, landing, and STOVL capability.