GrapheneOS aims to provide reasonably private and secure devices. It cannot do that once device support code like firmware, kernel and vendor code is no longer actively - maintained. Even if the community was prepared to take over maintainance of the open + maintained. Even if the community was prepared to take over maintenance of the open source code and to replace the rest, firmware would present a major issue, and the community has never been active or interested enough in in device support to consider attempting this. Unlike many other platforms, GrapheneOS has a much higher minimum