There's a lot to consider here:
- frictional durability (ie, how well will rubbing parts wear)
- impactive durability (ie, on the slide stop and where the recoil spring rests)
- tensional durability (stress areas, such as around the slide stop and on the rails)
- environmental durability (how well will the various parts hold up to environmental factors, like sunlight, moisture, and salts)
- design durability (the inherent characteristics of the design, i.e. what was the firearm designed to withstand)
These are all inter-relational, to some degree. For instance, environmental factors may result in a structural failure catalyzed by impact, or may speed the deterioration of wearing parts.
In most cases, however, the frame, slide/bolt, an barrel - the major parts of a firearm - will outlast the minor, smaller parts (trigger group, springs, bolt catch) provided there were no defects in the construction or materials.
Two case-in-points: I know of only two specific people who have shot more than 25,000 rounds in specific pistols. One was/is a stock Springfield GI 1911 (w/ only feed ramp polish and sight replacement having been done), which needed a recoil spring replacement at around 20k rounds (IIRC); that was in central Texas. The other is a H&K USP .45, which has had over 25,000 rounds put through it, and needed a new magazine release retention spring (it wasn't failing, it was just loose-feeling), recoil spring, and new barrel (which was partially shot out - a lot of hot ammo got put through it).
I also happen to know that there are quite a few aluminum alloy 1911s from the 1970s and earlier still floating around in use, though not nearly as many old steel ones. Proportionally, though, it would seem to be roughly the same as the all-steel.
The point is this: unless you are a "one pistol" kind of guy who shoots on the order of thousands of rounds through it a year, it will take you a lifetime to wear out a quality pistol - regardless of the material.
Taking exception for guns with 15,000+ rounds through the throat, those of poor design or designed specifically for a small number of rounds (Keltek P38T or what have you), and environmental factors (salt water, leaving a polymer gun in fierce sunlight for prolonged periods, extended acidic/close body carry, excessive over-cleaning), a gun is likely to outlast the owner and owner's child.