And you know why they last 4 years? But first, my first PC lasted 4 years. It would have lasted longer but I wanted a whole new system. Back to the question... they last as long as they do because to upgrade you have to buy a whole new system if ya want all the latest bleeding edge stuff. With a PC, you can piece it out. With both Apple and PC you can upgrade/update the OS as far as the hardware will permit.
The Power Mac G4 Cube was an 8"x8"x8" masterpiece of space efficiency. It was a powerful system, but it was also a powerful system with no expansion capabilities. Were the G3s upgradeable? Many of these fancy beautiful designs kill themselves because
of their design. A PC for all it's boxiness is still the easiest thing to upgrade. Pull the cover panel/door and there it is in all it's nekidness.
Change whatever you want and hope the OS doesn't scream bloody murder. Yeah the Windows OS has a lot to be desired for plug 'n play but I would rather yank my hair out trying to deal with a hardware problem than shell out another paycheck because I thought the Cube was pretty but I was blinded by the shine and afterglow of smooth round plastic and didn't care that it was not expandable. Now the iMacs, G3 G4 and G5s began to be a easier animal to upgrade but didn't it take a while? The G3 all in ones, hey we can upgrade the RAM, oh and the HDD.
Now giving a brain transplant to a G3 to G4 status is a neat trick. To give your iMac a genuine Motorola G4 brain, all you have to do is send in your old processor board in working condition to a variety of companies, such as FastMac. You get a new processor and new, faster backside cache, using your original board. Hell, with a PC ya pop it open ya pull the CPU and ya pop in the new one. A BIOS update first is always best to make sure that yer MoBo will play nice with the new CPU.
I can go on and on, but it is all preference. You can use whatever ya like as long as it works for you.