This is not a complete answer, but they could still have a proprietary backend, yet give people a lot more trust by having open-source client applications. The

This is not a complete answer, but they could still have a proprietary backend, yet give people a lot more trust by having open-source client applications. The idea being that forking would still require someone to build their own backend, making it only interesting for people that are willing to do a lot more in-depth engineering than just running an application.