It's both.

The ymmv is always big with electronics. I.e. my second last graphics card (gtx970) died within 2 yrs. But the aging effect from running 24/7 under high load is very real as well.

The high temperatures of permanently running on full power just wears it down

Besides mechanical parts you can still see failure from weary chemical components (capacitors, easily fixed), solder joints (fix depends on the joint. BGA chips aren't easy to fix) failing due to cycles of thermal expension and retraction or water damage, and lastly electromigration (the real unfixable damage, it usually happens from overvoltage/overheat).

I've seen every single piece of equipment worn out over the past decades except the CPU.

Hard disk and GPU are the worst in my experience. They have the shortest lifespan and they're the most impactful when they die. Hard disk die suddenly destroying all data, self explanatory. GPU die slowly, progressively having rendering error then crashing the system erratically.

I've taken 3 GPU to their grave between my personal computers and my family's.

Storage fails all the time. Cables, power supplies, UPS, monitors, keyboards, mice - I have a graveyard in a cabinet.

Also motherboards who's capacitors dry up; connectors that develop faults.

This is of course leaving out devices who's firmware get exploited and become poison (routers, USB devices with malware)

I've had hard disks fail in a PVR setup (heavy load), I've had memory fail, I've had motherboards fail, I've had power supplies fail, and I've literally typed laptop keyboards to death in ~2 years. I'd be utterly unsurprised about a GPU failure.

The best case scenario is "selling it because I'm buying a new one", the worst case scenario is "selling it because it's starting to flake out", and "selling it because the warranty ran out" is not much better.

How did you cool them? I mean, didn’t you find stock fans way too weak especially had it been in a “quiet” PC?