Aaaaargggh....
For my own purposes, I could happily do most things I enjoy on a 10 year old computer.
Trouble is, my day job is features training and IT support for a small veterinary software company. Dual platform, Mac and Windows. I've been doing this for the last 10 years, and over the last 5 years or so, annual OS updates on the Mac side have been giving me a massive compatibility wedgie.
For a given version of our database engine, many of our clients are down to a two or three year span, in terms of which version of OS X or MacOS will run their software package.
I've taken to running old operating systems as virtual machines in VMWare. Otherwise, I'd need 3 or 4 Macs plus a Windows computer at my desk, just to do features support....