> What are your thoughts on Microsoft Pluton and Google OpenTitan as TPM alternatives/emulators?

I am not familiar enough of the technical details of Pluton or OpenTitan to make a meaningful statement on their security.

> Should system attestation roots of trust be based on open-source firmware?

Yes, and not only root of trusts, I am strong believer in open source firmware in general. I have been developing coreboot as a hobby for a long time. I wish their was more industry support for such things, especially at the lowest levels of modern systems.