It's like asking someone when should you replace a car, it all depends, some people will keep a car running for 10+ years and just use it to go from point A to point B, others will want or need a newer car with GPS, road side assistance and mp3 capabilities...really comes down to your personal situation and preference.
I don't think it's as simple as that anymore. It used to be the case that you were either a 'power user' or 'non-power user'. A power user needed a high spec machine to use Adobe products, edit video, perhaps some 3D, etc. A non-power user did a bit of light word processing, email and web browsing, perhaps a bit of photo editing with a consumer package.
The development of the internet has changed that. Web apps and online multimedia content now require a fairly hefty spec in order to run acceptably. I eventually gave up on my G4 PB because web use was becoming particularly tedious and sometimes, in the case of online streaming video, nearly impossible. Photoshop, Illustrator and other 2D design apps are not really the limiting factor.
For that reason I don't quite agree with your analogy. A car will always do the humdrum, i.e. get you from A to B. I drive a 33 year old vintage Triumph and I get about just fine. Humdrum for a computer is web browsing, yet as stated this now requires a modern browser, up to date Javascript, the latest Flash player, reasonable graphics capability, and a decent processor. A five year old computer just doesn't provide that unless it was
very high spec. to start with.
Yes there are still 'power users' out there, but ordinary folk now need better kit than they used to be able to get away with, if they want to enjoy the content and tools available on the internet.