The small market share of old device isn't a given. There aren't few of them out there because we, the developers, make them unusable and people have to switch.

Most people don't care about having the latest specs, they want the latest features. If new features only ship to devices that are < 2 years old (like Android updates), older devices will quickly become obsolete. The hardware is still fine - I'm writing this on an almost exact 7 year old OnePlus 3 running a barely year old version of Android and the vast majority of things perform perfectly fine. Even the camera is perfectly fine even for modern standards now that I've switched to a modern camera app by tricking it into thinking my phone is a Pixel.

If we stop making old devices obsolete, dealing with old devices will be a lot easier.