Intel ME (which also has a "backdoor") is exactly what you are asking about. So, you can disable the ME now via some creative hacking of the ME firmware (you couldn't for like a decade before this, since like 2008), but AMD's equivalent, the PSP, might not have the capability to be disabled in a similar fashion. It was documented that the Intel ME could be disabled because the NSA wanted it that way (this needs verification, I'm sure). But anyway, whoever it was, they wanted it, it was implemented, and now the public has this capability. Then we got the ME hack...
We just need similar for the PSP (oh look, apparently according to this CTS news a reason to disable is present), and a big part of this problem is solved.