The F-35 has a much faster response time than the A-10--and, more importantly, survivability. In a contested environment, "suboptimal" CAS from an F-35 dropping precision munitions is much better than none at all (A-10 would get shot out of the sky by any near-peer before it could approach the theater). The A-10 also has high maintenance costs and a relatively low loiter time. For fighting terrorists in flipflops, something like a super-tucano does its job for a tenth of the operating cost. Fundamentally, the A-10 was not built for CAS. It was built as a last-ditch, suicide strafer of Soviet convoys during a land war in Europe. Very few were projected to survive past the first week.