CAS needs to have a direct-fire weapons capability to be effective or it becomes excluded through RoE. Paveways and some other PGMs can take an eternity to release if a targetting board has to sit within the RoE. Some PGMs like Hellfire or Brimstone are deemed direct fire, but they are way more expensive than a gun or CRV-7. So using F35 as your CAS asset is very expensive in both terms of operating cost, weapons cost and also the chance of total loss of a very expensive asset.
Apache has done a pretty good job as has Reaper, but as said, they are both slower than even an A-10 (that is about 150-250kts slower than a normal fast jet). The A10 does have a good loiter time over target, better than any fast jet (except B-1, but that has poor lookout), but it is about 8 hrs less than Reaper. As ever in aviation, the A-10 is a compromise, and a very good one at that, which is probably why the best replacement for an A-10 is another A-10! If the Cessna Scorpion gets a proven gun capability, then it might be in with a shout as it is pretty close to an A-10.
F35 is not the answer to everything, just like the F16 wasn't. It comes pretty close, but there will still be a need for niche capability airframes like A-10, V-22, Apache, Reaper, etc... etc...
LJ