Other relevant answers might include "true fault tolerance". More than half of your mainframe can be burned to a crisp, or being chewed on by a dinosaur - doesn't matter, it will keep working. The closest bad analogy I can think of...is a mainframe is like the NASA space missions of yesteryear. Every component has three or four duplicates, just waiting for a primary component to fail so they can take over. They aren't cost effective, but they are operationally effective.