Bus Factor
How many people could be hit by a bus before the project collapses. A bus factor of one means a single human holds it all. Morbid framing, essential metric.
Heard in the wild
“The payroll integration has a bus factor of exactly one, and he's interviewing.”
Tone check: Danger
Every org has a bus-factor-one system. Documentation and cross-training are cheaper than the alternative discovery process.