JF makes a good point. Fighter performance (9 g and supersonic) results in high cost.
The practical counter-argument (in many years of following debates about A-10s, Su-25s and more or less hypothetical things like A-16s, Mudfighters, SABAs and Rutan-Tutan Areses) is that you have to have fighters and they can do CAS, so why not use them?
The argument goes back and forth with technology and circumstances. Today's targeting pods make fast-jet CAS more practical, but a long campaign (as opposed to a rapid intervention) gets very expensive indeed, so you have an economic argument for a low-threat, low-cost aircraft.
And by low-threat I am thinking that the bad guys have nothing more serious than MANPADS and AAA. Once you get up to the Pantzyr level, it gets more serious, and besides, someone with that sort of kit may have other things that cause you problems.