The reason I asked was, in the present (and future) climate of defence expenditure scrutiny, why would the USAF want (or more to the point NEED) hundreds of F-35s when the USMC (Air) would, no doubt be the ones providing the majority of CAS?
Because the USMC is quite small. They have not and will not provide the majority of CAS -- there simply aren't enough USMC aircraft or aviators to do that.