@MartinBe @HistoPol @smallcircles
Threema "works", if you mean technically. yes. There is also a desktop client.
The app source code is available: (unfortunately on github) github.com/threema-ch/threema-

As to whether it "works" socially, there is an extra barrier to trying to move your network there, which is that each user has to buy a license (for less than the price of a beer in many countries) and this seems to be a huge barrier for most people who prefer their chat apps to be "free" 😉