> Do you want to fly on a plane where the flight control software was maybe overwritten?
I don't understand it. Whoever owns the place can replace any part of it, including computers. So being able to overwrite software doesn't change it. Furthermore, plane computers are not a consumer hardware.
You could make a better example with patched car software.
> The problem is Google and Apple locking down their Operating System, this is not a technical limitation on hardware.
The initial ROM bootloader contains hard-coded signature which prevents you from replacing Apple/Google software.
On pixel devices you can add your own signature to be checked and thus can use secure boot with a custom OS - that's how GrapheneOS works.
No need to strip out every wall, we just have to think about the problem and put doors at necessary places so we can enjoy both freedom AND security.