Secure Boot, or similar bootloader root of trust schemes, is the reason millions of phones and tablets will never run anything other than the manufacturer-provided OS and not what the user wants to run.

Yes, of course. I agree 100% that the designed-to-be-ewaste market is terrible. If I had my way, any manufacturer-privileging signing scheme would be illegal. So would the anticompetitive bundling of software with hardware devices, for that matter.

My point was that the threat of prohibiting libre Linux isn't from all manufacturers deciding to lock out installing Linux on their devices. But rather from remote attestation making it so that Google (et al) are able to force you to run a locked down operating system as a technically-enforced condition of interacting with their servers.