> a turboprop light attack aircraft armed with smart bombs can play a similar role as an A10 for CAS but significantly cheaper and with a longer loiter-time to boot.

Well, if you ignore how much quicker it will be shot down (in any environment where at least one of the two would be usable at all against combat forces), sure.

OTOH, the set of environments in which even the A-10 is usable is only going to shrink over time.

The point is if you're in an environment the A10 survives in, so would the prop aircraft. If you aren't, neither works anyway. The A10 exists in a middle ground where it is much more expensive but not really any less vulnerable regardless.