While engineers can make modifications to designs and select specific materials to better meet the demands of expected operating conditions and situations over the course of an expected service life, I'd think that all-steel guns would still have an ultimate advantage over those with alloy frames. Whether most owners would ever shoot the guns enough to realize that advantage is open to debate.
Proper use of alloys and careful attention to maintenance practices (lubrication & spring replacement) can help prolong the service life of alloy-framed pistols, too.
I remember back in the late 80's when there was some testing done by the FBI to determine whether alloy-framed pistols would be suitable for extended service in LE roles. (You have to remember that back then the typical service life specification for a military pistol was 5,000 rounds.)
Once it was observed that some of the alloy-framed pistols of that time period (including some 459's & 226's) exhibited cracked frames within 10,000 rounds, some of the firearms manufacturers responded by making changes to their alloy-framed pistols so they would provide longer service for LE use. We've all benefited from being able to buy improved alloy-frame pistols with much longer expected service lives since that time.
Steel frames were found to last significantly longer, though. No real surprise.
The use of different plastic formulations for pistol frames resulted in the use of steel frame rail inserts and reduced the contact surfaces between the frames and slides. While the steel inserts could provide a good service life, some changes in the shape, length and manufacture of the steel frame rail inserts continued to occur to address potential user issues. Also, even though the frame & slide rails were steel, manufacturers looked to find ways to better disperse and manage the recoil forces being experienced by the rest of the plastic frames (locking blocks, locking block pins, steel frame inserts at critical areas of stress, etc). Slide mass and recoil spring calibration became a fine dance.
Plastic frames can withstand certain forces very well and provide for useful service lives ... although I'd be less inclined to use one to hammer a wanted poster to a wall or post.
Joking aside, they can chip or perhaps suffer cracks more easily than metal under some conditions. The selection of plastic is carefully made taking into consideration its inherent advantages & disadvantages for different uses in firearms.
Nowadays we're seeing designs which allow for the replacement of steel frame rail inserts by way of replacing modular components. This allows for frame rail replacement, if needed, without having to replace the frame.
One of the newer designs, the M&P pistol being produced by S&W, even has stainless steel inserts molded into the frame to form a sub-chassis. The sear housing block and locking block (which contain the frame rails) are mounted within the frame and secured by steel coil pins which pretty much makes for a solid steel 'box' to absorb and mitigate recoil forces.
The most I've ever fired through an alloy-framed pistol was in a 9mm, one of the very production 3rd gen 6906's. I estimated I fired upwards of 45,000 rounds through that particular gun. It involved replacement of springs and some other parts over the course of that time, of course, and the frame was starting to look a bit worn toward the end, but it was still functional. I remember talking to some at the factory about it one time, and the fellow just chuckled and said that back when that gun had been produced they had never thought someone would ever try to shoot that many rounds through one. he also said that if I eventually managed to wear it out, that as an agency-owned LE weapon they'd replace it with something else (S&W offered their lifetime warranty to LE customers before it was offered to the general public). No need, as we were going to be replacing all of the older 3rd gen weapons with new ones, anyway.
There's another retired gentleman who posts on some of the firearm forums (can't remember if he visits this one, though) who has related how a friend of his, a former firearms instructor for an agency who used S&W alloy 9mm's for about 30 years, logged over 50,000 rounds of 115gr +P+ loads through an alloy-framed S&W. He reportedly kept the gun upon his retirement and was continuing to shoot it. I'm going to make a guess that he inspected and maintained the gun as recommended by the manufacturer.
I've fired several thousand rounds through a number of other alloy S&W's. Just nothing approaching the amount I fired through that one 6906. I examined a 3913TSW which experienced a frame crack in the front of the dustcover, which I was told by someone at the company was considered odd (and not in what they considered a critical location), and they cheerfully replaced the gun with another one for the owner. The owner estimated he'd fired between 12-15K rounds through the gun before I noticed the small crack (he thought it was a scratch). He's probably exceeded that amount in the replacement gun by now (he shoots a LOT) and it's been fine so far when I've periodically examined it. I've also given him a goodly supply of recoil springs and made sure he replaces them periodically, too.
This same fellow has fired more than 50,000 rounds through a pair of SW99's, too, one each chambered in 9mm and .40 S&W. Aside from an ammunition-related problem in the 9mm early in its life (a case head failure/blowout which damaged the frame and was replaced by S&W under warranty), and a broken ejector in the .40 at just over 50,000 rounds (which required a new sear housing block because the ejector is molded into the housing block, and which was sent to me at no charge as a warranty replacement part even with such a high round count) ... the guns have run well and long with only periodic spring replacements of various sorts.
I expect his SW99's to be running for years to come. I remember being told of a SW99 used in the factory's training academy which had been intentionally run without cleaning and lubrication, and no replacement parts, in excess of 75,000 rounds (last I heard). I just keep the fellow in replacement springs and make sure he cleans and lubricates the guns.
Oh yeah, the same guy has an all-stainless Colt OM .45 which was used to shoot over 20K rounds after it had been refined & tuned a bit. During that time he cracked a barrel bushing at over 10K rounds. He retired that little steel .45 to start carrying some alloy .45's & 9mm's as well as the plastic 9mm's & .40's.
I've got more than 10,000 rounds each through some small plastic guns (G26/27 & a SW99 9c) with only some spring and assorted small parts replacement. I expect them to run for some time with normal preventive maintenance practices.
Now, after all of my babbling, my point is that while all-steel guns can probably be expected to outlast most folks who own them, even the better quality alloy & plastic-framed guns have received some design and materials improvements over the years and are probably going to outlast most folks who buy and use them.
Of course, when you consider that the so called 'average' handgun owner is still considered likely to fire less than a couple boxes of ammunition through their handguns over the course of owning them, that's probably not a hard trick to manage.
Hey, just my random thoughts. As a firearms instructor and armorer of some small experience I've had a chance to experience, observe, be told about and read about things related to this subject from time to time.
I may still have a personal reference for an all-steel pistol, but I've long since accepted that alloy & plastic guns can be run hard and long ... as long as they receive the recommended maintenance. I certainly own and carry enough of them nowadays.
Enjoyable topic and discussion.
Remember when folks still hadn't accepted the "Coltalloy®" frame on the Commander? I think I still have a gun magazine buried somewhere with an article of an endurance test done by one of the popular writers of the 70's where a 'lightweight' Commander was successfully used to shoot 5,000 rounds. I bought that magazine because I listened to some of the nonsense about fears using aluminum Commanders for a lot of shooting. I eventually succumbed to gunshop myths of that time and traded it off for a steel "Combat" Commander. I plead youth and inexperience back then ...
