Like I said, firmware updates for the ME and PSP are generally tied to system firmware updates, so it's not just a matter of Intel and AMD packaging stuff - they'd need to change a lot of development methodology to ensure that these updates could be decoupled from the board vendor. And as far as Microsoft requiring that they implement a TPM - that's basically what they did? Microsoft just provided an implementation for them to use as well.
Pluton can be used in different contexts, and it can certainly be used in more IoT focused scenarios. UEFI doesn't really integrate with the DICE case terribly well (I'm dealing with DICE at the moment professionally, because I've made some poor choices in life), so I don't imagine it'll be relevant in the general purpose computing segment.