[spookylukey]

You need to be more specific. In the face of what kind of disruptions will it continue? For example:

* do you need to keep supplying electricity?

* do you need to keep paying the bills so that someone else will supply electricity?

* will it continue working if there is a hardware failure? What kind of hardware failures can be tolerated? Do you need to intervene if the data gets migrated to a new system?

* will it continue working if there is a flaw (not just a bug) found in something? e.g. it uses TLS version X which turns out to have some flaw in Y years time

* what about it society changes in some way so that the load on it is quite different in the future?

* how long are you expecting to do zero maintenance? At some point the operating system you run on will be obsolete, and there will be no upstream fixes to critical security bugs - what will happen then?

[_pdp_]

I should have been more clear but you have some excellent points that make me thinking. I think the crucial elements are to define the timeframe and usefulness which is somehow related to time.