Ever wonder what happens to a piece of software when the people who wrote it just stop showing up? In the industry, we call this the bus factor. It is a morbid name for a very simple metric. It measures how many key developers would have to be hit by a bus before a project becomes unmaintained. If that number is one or two, you are looking at a single point of failure.

Source: The Two-Person Wall: Why the Linux Backbone is More Fragile Than You Think