Final, final, final conclusion: Interacting with a computer makes it networked even if you're not intentionally using traditional networking technologies (TEMPEST attacks, arbitrary code execution through direct user input, etc).
Final, final, final, final conclusion: due to the complexity of computers, the only reliable way to achieve a moderate security in a system is to prevent it from being powered on.
The concept of C-I-A addresses this. Confidentiality, Integrity, Availability. If a system is not available for use then all the confidentiality of communications and integrity of data is useless.
"Pioneered method of keeping restrooms clean by keeping them locked during business hours."
Physical access has always been game over. Having a networked computer means your threat model is literally everyone on the planet, which is a much bigger problem than keeping people from physically getting access.
Direct physical access by the attacker isn't strictly necessary (i.e. operation Olympic Games) to "network" a computer you otherwise believe isn't networked. Unless you're bootstrapping from nothing attackers have tons of potential "ins" (firmware, the operating system, application software) to introduce backdoors or side-channels.
I've very nearly reached the point of just assuming all "modern" computers are effectively "networked", even if only by ultra-low bandwidth, exceedingly high-latency unidirectional side channels. Just bringing an "untrusted" computer into proximity of a "trusted" computer (say, having a smartphone in your pocket) might be enough to allow for exfiltration of data from the "trusted" system (assuming there's a side-channel in the "trusted" computer you're unaware of).
Ooh! This is a fascinating approach. I'm still skeptical that this is widespread enough of an issue to warrant the same level of caution as connecting a computer to the Internet, but I'd love to read more about examples of this actually happening in the real world (ie not researchers with full control of the environment) if you have any.