What you are saying is sound, and I agree it could be done.

But there are multiple caveats: - How do you hide the secret so that only "legitimate" operating systems can use it for establishing their sessions and not "Mate's bootleg totally not malware live USB"? - And unfortunately current CPUs don't implement this. - Additionally don't be so smug to think you need to decap a CPU to extract on-die secrets. Fault injection attacks are very effective and hard to defend against.

I agree the security of this can somewhat be somewhat improved, but if you are building a custom CPU anyhow, you might as well move the TPM on-die and avoid this problem entirely.