You can host your own repo (or someone else can) and just add it to the app

Adding a new source for every app becomes tiresome, especially when switching devices.

I do see potential for a closed source repo managed like an app store, but adding a repo for every app adds unnecessary security risks (as the app specific repo can contain any number of apps).

Hmm, maybe someone could start a F-Droid repository that accepts nonfree apps from everyone.