Intel CSME (formerly ME) and AMT ST (formerly PSP) have many properties that are / would be desireable for someone who is/was building a backdoor (DMA, TCP/IP stack that can use onboard ethernet without the host OS being aware, closed source, unremovable, etc).
Keep in mind that even if there has not been a backdoor deliberately added, there absolutely have been serious exploits in ME and PSP, and even for those who are not worried about the government, these security coprocessors still pose grave risk to data privacy and security, potentially handing "ring -3" access to non-government hackers.
While I personally share your belief that these are probable backdoors, it is important to remind skeptics that they pose a grave security and privacy threat even outside the context of a government backdoor conspiracy.
There's a great talk on the risks posed by the capabilities of these security coprocessors by an extremely well-versed individual at Google who was (leading? involved at a high level?) working on Google's efforts to neutralize these dangerous subsystems at Embedded Linux Con in 2017: https://youtu.be/iffTJ1vPCSo
Keep in mind that even if there has not been a backdoor deliberately added, there absolutely have been serious exploits in ME and PSP, and even for those who are not worried about the government, these security coprocessors still pose grave risk to data privacy and security, potentially handing "ring -3" access to non-government hackers.
While I personally share your belief that these are probable backdoors, it is important to remind skeptics that they pose a grave security and privacy threat even outside the context of a government backdoor conspiracy.
There's a great talk on the risks posed by the capabilities of these security coprocessors by an extremely well-versed individual at Google who was (leading? involved at a high level?) working on Google's efforts to neutralize these dangerous subsystems at Embedded Linux Con in 2017: https://youtu.be/iffTJ1vPCSo