please explain to me again about this so called "widely accepted concept", does a 5 years old mac retain its value better than a 5 years old pc? and exactly why do you think so?
Spend 10 minutes on eBay and you'll figure it out
PS, Let me know your 5 years old leopard machine, spec and detailes are appreciated.
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Not me but here's a writeup about it
http://blog.washingtonpost.com/fasterforward/2007/11/looking_into_leopard.html
Since so many of you asked, I put Leopard on a computer that barely met its system requirements--a Power Mac desktop with an 867 MHz G4 processor. (A sticker on the back reported that the Post bought it on July 10, 2001.) Here, the install took almost two hours--but the computer was surprisingly responsive afterwards. Leopard did shut off some of its flashier visual effects, though; for example, the background of the Time Machine interface was a static picture of stars instead of the usual animated starfield.
There have been several other published accounts of Leopard actually running better on old hardware than Tiger did.
As to your original post: Because the market allows for it. Honestly if any of the Macs in Apple's line up meet your needs exactly then the hardware is a solid value. If they don't, then yes you'll be paying more for the hardware just so you can run OS X.