My desktop PC, which is over a decade old, performs fine under current releases of Linux and Windows. My laptop PC is a bit over two years old. Performance is lacklustre under the current release of Windows. Newer isn't always better, particularly when you're comparing laptops to desktops. I also doubt that many laptops would have an equivalent lifespan to a desktop. They have more perilous lives due to their portability and portability means they are frequently built with more compact and less robust components. That is especially true when you consider that laptops are more difficult to repair (due to more integration and more difficult to obtain parts).
Granted, the iMac G4 is in a different league since it is a 20 year old machine that was produced when performance jumps were more meaningful. I can see it being a useful machine for someone who has limited interest and use for computers or simply wants something disconnected to get work done. I took the latter approach in the early 2000's and it worked quite well. (The fastest machine I had was a 68040, and I did run NetBSD on it.)