Why use old computers?

Moore's Law suggests processing power double every 18 months or so. Over time new generations of computers can do stuff more quickly. These gradual changes allow Software and Web Developers to pass more processing load onto the new machines.
Old machines, on the other hand, become less and less able to cope with the attendant higher processing demand made on them as time progresses

He 'tech landscape' sppeds up. So over time the demands that software and other hardware makes increases, while your hardware remains the same. So if you buy a PC and never ask it to do anything other than what it could do very well on day One then theoretically it wouldn't slow down.

Thing is programs get updated, and the version you were using is no longer supported, so you install the new version and that means your PC takes ahit. Over the years it mounts up. The iPhones were Apple actively making batteries die more quickly, supposedly this was to help consumers (reach in their pockets and buy another iPhone). Google isn't doing that, they are just not supporting Chromebooks after 6.5 years after they first came to market (not when you bought it)- that said a 6 year old battery would be on its last legs so probably dead.