The old ones don't get obsolete
This is a wonderful time in desktop hardware, where virtually any machine you find that was built in the past ten years will work for a lot of use cases. I do some volunteer admin for a charity nearby, and they frequently come to me asking me to reinstall or update their desktops. Before when I'd did this, I had to deal with machines that would not have much life left--not because they were physically broken, but just because they no longer provided enough power. I haven't done that in a long while. Every machine is capable of handling Windows 10, office, and a web browser. That's pretty much all the people do with them. So they haven't bought any new desktops for at least three years and many of the ones in service are much older than that. The same is true of laptops to an extent--the processors, memory, disks, and graphics continue to be reasonable for quite a while. Laptops are getting better in the realm of size, weight, and screen resolution though. Still, people can use their old machines for longer and in the case of desktops, there seems to be little likelihood that this will change soon.