Realistically, people who won't spend money to upgrade their phones aren't going to spend money on a subscription either. A different equitable solution is to make the app free but only support recent OSes; people running older OSes can use old APKs.

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.

Maybe, but the GPL license is suggestive that the author started and continued to work on this project because it was seen as a genuine benefit to others, and for many years that was enough to keep them going.

Sure financial incentive would have greased the wheels, but it might have undermined the spirit of the project in the first place

People generally pay for products in order to get a benefit. It's still genuine.

The difference is the seller gets to share some of the benefit.

The human mind is funny this way: sometimes getting small amounts of money is worse than not getting money at all.

https://en.m.wikipedia.org/wiki/Overjustification_effect

FWIW the author did previously take donations.