It’s a whole lot more than just firmware, though. The UI is proprietary. A bunch of the basic system apps are proprietary. The Android compatibility layer is proprietary. You’re not really buying into an open OS that happens to have proprietary components; you’re getting a proprietary OS built in open components.

A lot of people who are eager to install Linux on their phones this early are out off by the proprietary parts of SailfishOS. I personally want a Linux phone in large part due to distrust of proprietary software for a device with such capacity to invade my privacy.