I'll say it simply then.
You can't crack a system that utilizes a volatile keystore, once embedded in the part the system is setup to destroy the keystore, as any attempt to read back the key through software or hardware probing results in destruction of the part.
You can say "yes you can" till you're blue in the face, but you simply cannot. The technology isn't even currently in use because the process is deemed too expensive to use for production, hence theoreticals.
And with that we're done.
p.s. Your nuclear power station example still relies on the fact that the system architecture is a standard one and the payload for the exploit had been tested to actually work, and that there was a way to introduce code by the end user in the first place, in my theoretical I stated no access from any external storage, so it again doesn't hit the mark.
And no, using social engineering or bribery to get a chip specific keystore key isn't hacking the system, the theoretical was "can you make unhackable hardware", getting the key unique to your chip from the manufacturer is not hacking, in the same way that using a set of keys for opening a door isn't lockpicking.
And even if you had the chip specific unique key, you'd need to know how to actually use it, to put it in another way, imagine a large indestructible safe, it has a super complex unpickable keyhole, a handle and several dials with inscriptions you can't even read, even if you get the key for the safe, you'd still need to know the exact configuration of the dials because each time you try the handle,the safe unlock mechanism disables and you can never again open it. You would then need a new safe and a new key for that safe just to try again. And each new safe has a different arrangement of symbols on the dials.
That is basically what you would be up against.
its unhackable, but also impractical because any deviation from standard operation (software crash, bug, mem leak, etc) would trigger the keystore burn, so one slight fuckup in a games code and the system would essentially be a paperweight (this is another reason it isn't used).
