[dessant]

Maybe it's not your intention, but from your comments it comes across like you're putting very little value on other people's work. It was probably a substantial effort that took years to bring these projects to the usability and popularity that they achieved. Again, i'm not sure why did the user above find it problematic to have a paid version of the app on Google Play, and the same version with the same features distributed for free on F-Droid.

[dun44]

Oh, I'm sure writing those apps was an effort. But I was asking about something quite different - the cost of maintaining them. You do need time or money or people to keep an Open Source project alive; but you don't need to pay the authors for the work they've done previously.

As for the problem with a paid version - it's because when it happens, shortly afterwards the free version starts to rot. Monetisation might be good for the owner, but it's almost always terrible to user experience.

[dessant]

From a quick look on GitHub the projects were not just an initial effort with little time invested in after release, but were actively developed for years. This year alone there are several releases which contain new features.

If I'm not mistaken, they were publishing the exact same version on both app stores, and the apps could be purchased or some of the features were put behind in-app purchases on Google Play. This is a reasonable funding model that has allowed many open source apps to thrive, though in this developer's case maybe it did not meet their needs.