CzCrimpy Very few people have an Apple Developer account, so most users won't be able to build and install the app themselves.
True. Tvheadend has reimbursed developers for the annual (quite high) Apple Developer Account fee, so if someone really wants to pick up the abandoned work there's always a way. You can of course also start with a paid model and open source it later on if you no longer want to maintain it, or have it open source and paid (as users won't be able to install it themselves without the much more expensive developer account).
CzCrimpy and a clone of the app appearing on the App Store within a week, being sold for money by someone else.
Choose the license wisely and go after them. A single won lawsuit can bring more money than any donation or subscription model 😉 Also you can just file a DMCA takedown request if they are violating the license and Apple will remove the app.
I can only speak for Tvheadend, and in fact you can check yourself on opencollective for the exact numbers, but the monthly donations wouldn't be enough to make a living (not that anyone ever tried that, the money donated towards Tvheadend is used only for project-related expenses like Hosting, Domain, Apple Developer Account, Test Hardware and so on). Assuming that only a subset of these users will be using and donating towards an App, you'll likely get (significantly) less than that.