So what is the plan for handling the US nuclear warhead stockpile as the empire crumbles? I'm worried about billionaires with nukes. Maybe not the person directly but people behind all that envision super wealthy city-states and I totally expect those to have nukes.
The nuclear codes won't stop anyone with time and engineers. These are intended for physically arming the strong link in the warhead that is supposed to send the signal to the exclusion zone but someone with unrestricted access should be able to override it and send the signal directly. Although over the years the mechanical systems were replaced with electronics that eventually become encrypted microelectronics, IIUC the actual device that does the kaboom remained with its original design and applying voltage will be able to trigger it. Safe against rough handlers(i.e. crazy solders) but won't stop people with unrestricted access.