As a matter of fact I have just bought a new saw, a 440 and maybe you want to ask a member here called blsnelling what happened to some of my old ones and how people benefited from parts.
This thread
http://arboristsite.com/showthread.php?t=40520 and page 10 shows why and what happens. Maybe you need to see outside the boundaries of your confined imagination.
So, I have a new one now, and 2 old spares in the shed.
If anyone has old (non corroded) 440 cases I'd be interested, I could build a complete new saw.
Here's another simple incident. I accidently left a saw sitting ontop of my toolbox in my ute and drove off. Went around a corner and the saw flew off and smashed into the gutter, busted some covers, no big deal just put on the spares at home.
I get 12months out of a 440, about 18 months to 2 years out of a 250 ... 3 to 5 years out of a 200T and my 660 is 4 years old and my 460 is 8 years old.
You can see how I isolate damage to two certain saws, now a model change means a stack of useless parts on what are otherwise great saws anyway.