This.

A decade ago I was technical co-founder of a company [0] that made interactive photo booths and I chose CentOS for the OS.

There are some out in the wild still working and powered on 24/7 and not a peep from any of them.

We only ever did a few manual updates early on - after determining that the spotty, expensive cellular wasn't worth wasting on non-security updates - so most of them are running whatever version was out ten years ago.

Rock solid.

[0] https://sooh.com

You either need to upgrade or unplug (from the internet).

There are still places out there that are running WindowsNT or DOS even. Because they have applications which simply won't run anywhere else or need to talk to ancient hardware that runs over a parallel port or some weird crap like that. These machines will literally run forever, but you wouldn't connect it to the internet. Your hypothetical cave device would be the same.

Upgrading your OS always carries risk. Whether it's a single yum command or copying your entire app to a new OS.

Besides, if you're on CentOS 8 then wouldn't you also be looking at Docker or something? Isn't this a solved problem?