IME these systems can be quite fragile in practice. All it takes is one pre-signature exploit (like U-boot parsing ext4 and devicetree before verifying signature) and your whole chain becomes useless.

And while the kernel is quite secure against hacks from userspace, the hardware interfaces are generally more trusted. This is not a problem on smartphones or embedded devices where you can obfuscate everything on a small SoC but the whole PC/x86_64 platform is much more flexible and open. I doubt there is a way to get reliable attestation on current desktop systems (many of which are assembled from independent parts) unless you get complete buy-in from all the manufacturers.

Finally, with AI systems recently increasing in power, perhaps soon the nuclear option of camera + CV + keyboard/mouse will become practical.