Sure, but the NK and NV armies did have good anti-aircraft setups, likely much more effective than relatively untrained fighters with a weapon designed to shoot at other people with.

Richthofen in WW1 was brought down by a single shot from a rifle, but that is widely considered to be a freak shot.

Nevertheless, you're right that it is never safe to get in range of small arms fire, hence the genesis of the A-10.