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.