The increasing proliferation of ever-better MANPADS and radar-guided guns (like those of the ZSU family) has closed off the lower tier of enemy airspace to aircraft, regardless of how well armored they may be.

We’ve reached a point where, against even many third-world militaries, A-10-style armor suffices to maybe save the pilot and possibly even allows them to get the aircraft home, but the likelihood is that too many sorties will have to turn back before reaching the target to justify the cost of the aircraft. Even in the First Gulf War the A-10s got chewed up a fair bit, though 25 years ago things weren’t yet so bad as to render them ineffective.

From now one, CAS against all but the most primitive of enemies is necessarily going to have to be performed via smart-munitions (bomb or missile) from mid to high altitude. We’ve gained a lot of experience with that in Afghanistan and Iraq (that was mostly what the A-10s were doing there, too; the gun ended up being dead-weight for most missions), enough to see that it’s a workable method. There are some potential issues with using it against a more savvy foe, but there’s simply not much choice at this point.