Only the clients source code are available, so we know what data is encrypted before sending to server. The server side is closed source.

Yeah, but why would anyone use the competitor? Proton’s biggest advantages are its security and privacy. It’s hard to back that up with code and even more with plain words.

Having their code be open source is a very important step in maintaining that image, and that image is not something a competitor can just plagiarize.

In addition to what others have said, the license is GPL3, so a competitor would legally need to open source their app too.