If it's openable it's not locked down. If I can't trust the kernel then every level above it is tainted (this is assuming you can trust the CPU and firmware, but at least ARM is better than Intel here).
OK, let me be more accurate: "user-openable" in the sense that the user is free to choose to trust any OS vendor they want. What's the unmitigatable harm?
A user that isn't the owner of the device - say a repair tech, but possibly a remote third party - installs an OS they trust, which scrapes all the device owners personal info and sends it to their server. The device owner may not ever become aware of this.
For the remote third party case, the option to unlock should be isolated from the OS so that it needs physical access (boot to bios/recovery mode). AIUI that's how it's typically implemented in Androids and in Secure Boot.
The point about attackers with physical access is fair, though significantly mitigated on Androids (and Chromebooks I think) by a warning (of varying severity) that the OS has been modified. On my Nexus 5, it's a subtle "unlocked" icon, on my Moto G it's an unmissable red triangle on white background with "warranty void" and other scary words.
It's still vulnerable to the user getting a prebooted device and only seeing the warning on next boot, so not perfect, but also not the only imaginable defense. One could e.g. move initial authentication to the locked down "bios" phase so either (a) the warning is visible there and/or (b) the OS isn't replaceable in the first place without authentication.
And we should bear in mind that a malicious repair tech can probably convince a fair portion of the population to give away their password :/
