Sooo driving easy on your car engine for 500 miles does nothing for a car expected to go 500k miles?
There is two times in a mechanical objects life where wear happens fastest and that is at the start as things mesh together and at the end when it starts to fail failure. Breaking in is just wear fitting. So yes, there is some sense to it, but it's also B.S. but not for the reason you think.
Switches are usually lubed, which means break in will take longer than a week and not using at least some lube can actually cause more wear than actual break in but this too is not the real problem, the plastics used often have at least one plastic that is considered self lubricating. No, the biggest reason I say it's B.S. though is because the parts were worn in as they were. Breaking them in, then disassembling them messes with the fitting. Worse, most people don't do them one at a time and therefore they completely destroyed any match-fit that occurred. Switch A was worn in, but you swapped in part B's stem and the spring, which had no wear is now no longer sitting the same way it was. So what exactly is worn in together at this point? Nothing. And what if housing A has a slight switch, it has now worn stem A in a specific way but since you swapped that stem into housing B, those two parts not only don't match but are even further off than they were from the factory and will now wear at an accelerated rate compared to the rest, at least until they too settle but their life span has been compromised regardless. How much, who knows, odds are they will still outlast your expected lifespan.
Is any of it enough to matter, probably not if your switches are all still pretty new, doing this with older switches is much more of an issue, particularly in hot swap boards. Soldered switches can relax, they are not under any stress when put in place, hot swap forces the switch to line up the solder prongs and housing in a very tight manner not designed and this can lead to housing and leaf distortion, with light enough springs this distortion can be enough to cause bind. I said early on hot swap was a "hack", I stand by that even though I do use it, why? It's convenient, but I also don't expect my hot swap boards to ever be the next Filco, much less Model M, keyboards that can be passed on through multiple owners without issue.
By the way, most "breaking in" is actually just the lube spreading out and being redistributed and parts gently conforming to their new home, there's very little actual plastic wear during this time.
