Leaving aside the debatable nature of how open and free a RPi is, what's the point of "general-purpose computing" when it can't actually be used? When/if you need a locked-down machine controlled by some central authority to do what you can still do today with a free and open one, i.e. create and consume media, interact with services, communicate with others, etc., how much value does a truly libre computer have? It's almost like the "just build your own platform" argument when it comes to censorship.
Arm, including the quirky RPi, is not comparable in openness to x86 general purpose desktops, which were a happy confluence of accidents, determined individuals and scrappy businesses. That is still worth defending, if only to slow the slide backwards.
M1 Macs have a unique take on hardware security that can co-exist with general-purpose computing for open-source Linux, and Apple's vertical integration. https://archive.fosdem.org/2022/schedule/speaker/xeno_kovah/. Hopefully Asahi Linux will succeed in creating a relatively maintainable port for multiple generations of Apple Silicon.