When I went through an armorers course a hundred years ago, there was a section on the lifespan of pistol frames. An alloy framed Hi-Power had a service life of about 35,000 rounds while the improved M9 frame had a service life of 75,000 rounds. While the aluminum receiver of the M16 can often surpass 200,000 rds, pistols receive substantially more battering from the slide moving back & forth than the rifle. Alloy pistol frames often have their lifespan extended by inserting steel pins in high stress areas.
The service life of the steel framed M1911 improved from about 35,000 rounds to well over 100,000 rounds as we made gains in metalurgy heading into WWII and the newer M1911A1 production. When examined closely, most steel framed pistols had developed hair line stress fractures after just 35,000 rounds, though the frame may serve well into the 100,000 range without fracturing... or maybe fracture the next round fired. They last longer than aluminum, but... they still fail.
Polymer frames didn't have a service life estimate at the time. Shooting them seemed to have little impact on the cohesiveness of the polymer. What would tend to fail first is the holes where steel pins are inserted for the trigger and such. Holster wear and wear from inserting magazines were also taking a great toll on the plastic frames... but shooting them didn't.