bus factor

 bus factor  is a relative measurement of risk concerning a project or service's potential fragility judged by asking the question "Will a project (or service) fail if one of its members is hit by a bus?" Also known as train factor, lorry factor, and other variations.

How to mitigate
Open source is one way to mitigate high bus factor. If a project is open source, anyone can run it themselves, fork and improve it, and let existing users migrate to their new instance. We've seen this happen a bit with e.g. silo.pub and Woodwind.