Yes, PCs "break". Caps go bad, fans seize (without the owner's knowing), heat sinks become detached.
After repairing computers for 12 years, if you can think of something that's happened to a computer - I've seen it happen. The fact that you have 20 year old working computers doesn't suprise me at all.
I had a gentleman bring in a 486/25 SX (Circa '92 or so). Ran perfectly. No real dust buildup, all original parts. Windows 95 even booted up fine. But I have to tell you, that's a total and complete anomaly. The last time I saw a running 486 was about 10 years ago, and the last time I saw one in that condition was when I was still in high school in the early 90's.
Modern consumer-level computers tend to last about 4 years or so. Heck, even server-grade parts tend to fail with alarming regularity. Honestly, I don't think they're much better than their desktop brethren, just more expensive and oddly enough in more minimal (Re: Cheaper) packaging.