The major issue with US Military M9 "maintenance" is that it's almost nonexistent. A pistol is shot until the locking block breaks, then an armorer pounds the gun apart and just drops in another locking block. Adding injury to insult, this "new" locking block is often from a previously used gun that broke or was scrapped for parts, so the block isn't really new and you have no idea how many rounds it's fired already.
Over the course of thousands of rounds, a new gun's locking block and the slide (specifically, the front edges of the cuts where the locking block moves up and down) wear in together. Because of the barrel's rifling, the barrel torques every time it's fired so the wear is not even from right to left.
When you drop a different locking block in the gun, the shoulders of the block may no longer mate up properly with the slide cut. Now, every time the gun fires, instead of both shoulders getting struck by the slide simultaneously you get just one shoulder forced to absorb the full impact of the slide's recoil. So the block breaks much faster. This is why many people see their first locking block last a good while, then the next one breaks much sooner, and the third one sooner still, and so on.
A trained armorer with a file can make sure the replacement locking block and the slide mate flush, which will increase the life of the new block. The only downside to this is that (at least as it was being taught five+ years ago) the fitting was done to the slide, so if you replaced locking blocks enough times you'd eventually have slide cuts that were out of spec and the barrel lockup might suffer.
Furthermore, when the locking block is fired to the point where it actually breaks you frequently have damage to the aluminum frame rails and possibly other parts of the gun. So simply replacing the block doesn't address the stresses placed on the gun when the first block broke. It's a lot like a car tire. If you replace it on a schedule before it blows, you're just spending money. If you wait until it blows out, not only is it much less convenient (not to mention it can happen at a time when it costs you more than convenience) but you run a higher risk of damaging other components (wheel, etc.) too.