Stan By -- I think it's been mentioned here already, but perhaps not loudly enough: The problem last Friday was not with the new software release. It was an existing problem, always thought to be inoffensive until it chose to manifest itself in a new and exciting way.
When we install a new release, we in fact do exactly as you say: New and old co-exist on disk, and we can switch back to the old one "easily". (Actually, switching back involves the same impact to operations as the original switch).
But on Friday, it wouldn't have helped.
--Dinosaur