It would take an unusual amount of wear to significantly reduce the reliability of most firearms.
To a certain point, wear is good. It allows the mechanism to operate with less effort as the high spots get rubbed off. But, too much wear causes parts not to fit together properly, and that's where trouble begins.
Speaking of the M16 only, the "valuable part" is the lower receiver itself, and in normal use, the only parts of it that are in any danger of wearing out are the takedown pin holes and the fire control group pin holes, and bushings can be installed in them if they're worn too far, and the bushings are replaceable. Plus you can get anti-rotation fire control group pins, thereby removing that source of wear as the pins don't spin. They're linked together with an external strap.
CJ