It is going to be pretty hard to take an off-the-shelf notebook and make the proposed changes, especially when you are talking about implementing hardware kill switches for certain components on a 12 layer PCB. The strongest incarnation of this idea seems to involve cooperation with, and some degree of trust in, the motherboard manufacturer (whether that is Purism or someone else). Is there any solution to nation-state hardware attacks involving intercepting your notebook while it is being delivered to you? I think that has always been considered "game over" as far as maintaining security.
However, if you have that, you do get a significant benefit. You are no longer vulnerable to someone sticking a rootkit in your BIOS. That is where a lot of the up-and-coming SMM-level rootkits like to be installed. You can move a fair chunk of your root of trust (the firmware, etc) into a device separate from the notebook, and feel pretty comfortable that device is going to provide certain guarantees about flash memory contents that we do not have with today's systems.
However, if you have that, you do get a significant benefit. You are no longer vulnerable to someone sticking a rootkit in your BIOS. That is where a lot of the up-and-coming SMM-level rootkits like to be installed. You can move a fair chunk of your root of trust (the firmware, etc) into a device separate from the notebook, and feel pretty comfortable that device is going to provide certain guarantees about flash memory contents that we do not have with today's systems.