Why 5-6 years, that's about when I start seeing the cascade of little things. Weird transitory bugs when rebooting. Speed issues and compatibility issues with bloated new software etc. After that amount of time, I start to spend way too much time maintaining them.

I could tinker with them and keep them going. Its what I used to do when my kids were small. Install a Linux distro on an old computer, load a bunch of educational games and set the browser homepage to PBSkids.

However I have 5 computers to maintain now and my teenagers need compatible fast systems for college and school. My wife works from home at times and needs something that reliably works.