The A-10 is not the only aircraft performing CAS nowadays. F-16, Hornet, Super Hornet, F-15, Tornado, Harrier, Rafale, Mirage 2000, Super Etendard, B-1, KC-130 etc. have all been doing this in Irak, Afghanistan etc.
With the targeting pods, advanced radio's, ROVER equipment etc. they have done very well, using GBU-12, GBU-54 etc.
Then count in MQ-9 Reaper UAV, Apache helos, Tiger, AC-130 gunships and the likes.
Second, A-10 was a dedicated tankbuster, not a dedicated CAS aircraft originally.
Third, why would F-35 be bad at CAS? It has WILDLY superior optics/comms/precision avionics over A-10C with SADL and Litening II. The F-35's computers will have a HUGE computing advantage to programme laser jdam and other precision munitions.