My personal opinion is that Threema is still a viable alternative to WhatsApp. The app is open-source, there's even a 'libre' variant on F-Droid. The service is proprietary though. The jurisdiction (Switzerland) seems more trustworthy currently. I don't really understand the business model of Threema: It seems like they have some revenue from a commercial service "Threema Work". The initial cost for purchasing an app license does not really contribute to sustainability I guess. I suspect it is more for making scams more expensive. I hope they can be sustainable from that and don't need to start doing shit.