^Air Support Control Officer, USMC
features some contrived anti-air defense that is somehow not good enough to defeat the F-35s rudimentary stealth but is good enough to be a credible thread to the A-10
Pretty much any late-Cold War SARH (Semi-Active Radar Homing) SAM fits this category. A prolific example would be the 2K12/ SA-6 'Kub'. If the experience bombing Yugoslavia is any indication, some of them will survive SEAD missions. Not a contrived scenario at all. Also applies to any Third-Generation or even early Fourth-Generation fighters doing CAP missions. Sometimes there are Leakers (enemy aircraft that get past friendly air defenses/CAP).
doesn't require the aircraft providing CAS to loiter, expend large amounts of ordinance, use the main gun extensively, fly low/slow or do anything the F-35 sucks at
Loiter time: If you have a permissive air environment with almost-no air threat, which is the sort of scenario where you'll want to use the A-10, then the CAS platforms with the best loiter times are an MQ-9 Reaper and an AC-130 anyway. If you really want a fast jet with loiter time....the F-15E beats both the A-10 and the F-35...and is a better dogfighter too.
Expending large amounts of ordinance: the F-35A and C both have 18,000lbs max payload. The A-10 is 16,000lbs. The F-35B is 15,000lbs. So even if trucking a full bombload was required, this is a wash.
Use the main gun extensively: Of course the A-10's best feature is strafing tank columns with gun rounds, so not much argument there, but that itself is a somewhat contrived scenario. Here's two really good posts on F-16.net about A-10 CAS employment and modernization trends:
requires the CAS aircraft to sprint around at higher speeds than the A-10 is capable of
Sometimes time-sensitive immediate air support requests (say, for engaging High Value Targets on the move) are required. Blasting the target with a HIMARs fire mission would be even faster, but not a good guarantee of target destruction, especially for something mobile and armored...
reconstitutes the CAS mission to consist of dropping a small amount of ordinance from high altitude with no loiter
This is not a "reconstitution" of the CAS mission. CAS is not restricted to low altitude bombing/strafing runs. It has more to do with battlespace/command & control relationships. While flight behavior of the aircraft can be a limitation, that's very much dependent on aviation platform, the ordnance, and the tactical situation and is not specific to the CAS mission itself. Reference MCWP 3-23 Offensive Air Support Page 2-2: "CAS is air action by fixed- and rotary-wing aircraft against hostile targets that are in close proximity to friendly forces and which require detailed integration of each air mission with the fire and movement of those forces"