If a machine is running, say, windows xp, and it does in a manner that satisfies the owner's need, then it should be able to keep running forever in the same manner, provided that hardware lasts and/or is replaced with adequate spares, not necessarily anything newer.
It's ok if that computer becomes incompatible with newer software because of that.
It's ok if that computer becomes incompatible with newer outside/network services because of that.
It's not ok if that computer stops being able to execute exactly the same process that it could do 10 years ago.