Most parts I've replaced have been voluntary replacements, not because something broke or wore out.
And, sadly, almost without exception, the few broken/damaged parts I've replaced have been parts I screwed up myself.
Part of the reason I haven't broken many parts is that I have a number of guns, and that means that my shooting gets spread out over all my guns. If I had only a single gun, I would have put a lot more wear on that single gun and likely would had a lot more normal wear parts replacements by now.
Also, I occasionally sell or trade guns, and those guns then become additional firearms that I have fired that I no longer have responsibility for maintaining/replacing parts.
In addition, most of my guns don't get carried much, when they're not being used, they're well protected in the safe.
I guess what I'm saying, is that if I had a single gun that I used for training, carry and practice, I would have likely had to practically rebuild it from scratch by now. As it is, in my current situation, it's unlikely I'll ever have to do extensive parts replacement on any of my firearms unless I simply want to change things around, or unless I have to or choose to sell a number of my firearms.
Thanks all for the responses so far. I was looking to gather some perspective on the recent DHS parts order:
DHS does training for a lot of different federal LE agencies. I found an article awhile back that indicated they trained something like 70,000 LE agents from 90 different agencies in a single year.
Some of this training is probably a bit hard on the firearms. For example, I just finished a class, and one portion of it involved repetitively clearing double-feed jams for practice. The particular method taught is somewhat hard on the extractor but it is a very fast and simple process so the possible damage is considered worth the risk. The trainer stated that he had spare extractors on hand in case there was a problem.
Also, many times, these contracts are option contracts spread out over a number of years. It's not like they're actually purchasing all those parts, they are taking out an option to purchase UP TO that many parts over X number of years.