it's dense plastic, and if it lasts 40 years I'll be in my mid sixties when the gun goes out of service- I have others and will be too feeble to get much use out of most of them. any gun that is used hard and abused WILL go down for maintenace, if not BREAK, as that is how reality works. I assume that if a glock that is ? years old can handle 10mm and 357 SIG then 40S&W, 9mm Luger and 45acp users should hold up good in the long run.
I'm guessing that since auto "polymer" stuff under car hoods lasts quite awhile when exposed to random chemicals, often very reactive PETROLEUM based chemicals at that, 200 degrees fahrenheit plus, great humidity at over 200 F (which IS a known issue for glock frames, they soak up water if it's really hot), and plenty of neglect- that a firearm that is built to take quite a bit of battering, be exposed to gun cleaning chemicals, be a reliable duty weapon for huge markets, not be a liability due to kB risk, and keep the makers rich would be dependable enough to at LEAST take the same amound of stressors as the stuff under a car hood (which one never will under anything other than a deliberate torture test). The thing that DOES bother me about a glock is that a dog can chew it up, and abrasion of the frame is always possible.