I work in plastic processing. The more weather, shooting and wear the gun has seen obviously would deteriorate it. However simple time and oxidation, moisture does cause deterioration
If a gun is a safe queen that hasn't seen the light of day.....absolutely its fine, and Nylon generally is a robust polymer. But the additives and formulations, and processes in molding from the 80's to current time are night and day.
A frame made now has additives that strengthen, aid in flex, weathering, and process that nothing in the made in the 1980's would have. That said, there are plenty of manufacturers that push the envelope as far as part thickness and design go....P80 is one of them. Alot of them, just to make the frame cheaper with less material.
And yes, the old Glock frame was probably over engineered as thinking was it must be as strong as steel, and made as such.
And I'm not saying an old 80's gun is going to deteriorate to the point where it would fall apart in hand, but stress cracking due to polymer aging, and weathering certainly could be a factor.