Open sourcing the apps doesn't make them free in the app stores. But it means that anyone can publish a build for free, no? Either on GitHub or wherever, for people who can install an APK on their device. Or in the app store, as a rival "free Threema clone" or whatever (I guess one would have to be careful about trademark issues).

You still need a license to create new accounts on the official servers. So if you want to be in the official network, you have to pay.

If you want to be in the official network, yes. But like I said, open sourcing the client possibly open sources the relevant parts of the protocol that would allow compatible, maybe even federated, self-hostable and free servers.

Yes but even then only some users will use those. Hence, Threema will continue to generate income from the app stores.

Yes, but they'll obviously check that your account has a license before sending messages.

How does Threema differentiate itself from open protocols like Matrix or XMPP? Why might someone choose Threema instead of Matrix or XMPP?