Diginagain,
The Harriers suitability for CAS was provided, not by its survivability, but by it's ability to operate from close to the battlefield and therefore generate high sortie rates. No-one ever pretended that operating at low level in-close was anything other than an extremely hostile environment. The only measure of protection from small arms and hand-helds was provided by 500kts and 100ft. This was augmented in the GR7 by the ability to do it at night.
You will have noticed by the way the A10 is employed that even it will not loiter at low levels in a high threat environment and we have certainly learnt that Apache, for all its capabilities, is vulnerable to a farmer with a muzzle loader.