"Hey, wait: maybe I'm the only one with this relic of a computer, but I love it."
As long as the computer still does what one needs, that's good enough.
I have a relic that's even more of a relic: a Macintosh Quadra 650, made in the early 90s. It's been years since it was on the Internet (and never during the time I've had it), but it still works, and I still use it for certain tasks, like heavy duty word processing. It serves me well for what I use it for. And I do enjoy the reactions of others ("You're still using THAT thing?!?")
The computer I'm using to post this is the newest computer I have. I have no idea when it was made, but I think 2004 at the latest.
"Just shocked at how quickly they make 8 year old computers obsolete, no parts available."
This is a real problem and irritation. Apple seems to be particularly good at killing off older technology--although they are far from alone.
"Somebody has to give the computer industry a comeupance."
Somebody has. Actually many somebodies. There are developers out there who are still making software that supports various older computers. TenFourFox--which I linked to above--is one project that got spurred by the desire to continue using 10.4 Tiger on the Internet.
Linux is another example. With Linux one can have a modern, fully supported OS on hardware that isn't otherwise supported. The computer I'm using right this moment is running Linux. This computer's orginal OS (Windows XP) is still supported--but not for much longer, and I question if Windows 7 would run happily. (Actually I doubt it would run given that the memory is currently limited to 256MB.)
Unfortunately, though, the mainstream--both normal users and the industry--push a "New! is better!" attitude, and a preference for The Big Name (which may or may not be a good product--but certainly won't be a perfect choice for 100% of users 100% of the time).
Incidentally, since this thread was originally about a PowerBook G4, it is worth mentioning that Linux is available for older Apple computers. But the installation is not exactly a trivial matter. I installed MintPCC on one machine, and the experience was more primitive than what I remember from Intel installations--of 2005. Thus, it's not something I'd recommend for many average users.