This is actually supported by the Play store itself where you can serve older APK to phones with older OS.

I’ve no way to upgrade iOS beyond 11 and miss the stock software and more. Video editing currently not possible with stock app, nor SoundCloud. No OTA upgrade nor iTunes, until I have clean cheap power and strong Internet access. Used to get it at libraries until March. Had Android, liked APK distro, glad it still works. Very cool that the store offers it, but also that there are alt stores.

Why not charge for the app AND then with the income you have incentive to keep supporting old devices.

Because the money earned vs old devices probably isn't worth it. At some point you have to cut support on old stuff unless there is some financial pay off or you're retired and want a hobby to keep you busy.

Of course you could use revenue from paying customers to support moochers, but there's no real incentive to do so and it's arguably unfair.