> (Nearly?) All ChromeOS devices use CoreBoot. You really can't get much more open than that.
Last time I looked, it was really hard to install anything other than ChromeOS on Chromebook hardware. You can install a chrooted Linux on them, yes, but on the device itself you can't even execute unsigned binaries.
Impossible? No. Harder than executing an installer with elevated rights? Yes. Plus, they also come with pre-installed software like Google Docs.
> Android is open source and if you don't buy a locked down device from a carrier,
That's quite a big if. Android itself is open source, yes, but >90% of the ecosystem rely on Google Play services, which are anything but. And, when talking about pre-installed apps that the user can't remove without a lot of effort, Android basically invented that.
> the bootloader is unlockable and the system easily rootable.
If you wipe your device and void your warranty. And then install a third-party binary to actually use those rights, while similarly loosing the ability to use quite a few apps (like banking). That is, if the manufacturer makes it that easy (Xiaomi, for example, needs you to sign up and wait for that - it's possible, but anything but frictionless).
> Now, if you were to mention MacOS and iOS... then you definitely would have had a point
I can't talk about MacOS, to be honest. Though, as far as I know, getting a root shell is not hard and running own software is not a problem.
We agree on iOS, but the grandparent talked about PCs - iOS really does not fall into that category (that's why I explicitly mentioned smartphones).
> Your two examples of something more 'not my pc anymore' than Windows aren't exactly good ones.
Windows is not a good example of that. Don't get me wrong, I don't like windows. But it's by far not the worst example of a locked-down, vendor-owned system and it would be even less bad if the administration UX would be simpler.