> either some update goes badly wrong [...] or the system just point-blank refuses to update

You don't understand, in that post I'm talking about purposefully not updating. At all. Updates can't go wrong if they don't happen. If it refuses to update then you win.

> What I want is something that's more like an appliance

Yeah, you get it! Set it up and then leave it alone.

And don't expose it to the internet.