I think you nailed the crux of the problem. The challenge for engineering management is how to provide metrics to measure your bus factor reduction efforts and

I think you nailed the crux of the problem. The challenge for engineering management is how to provide metrics to measure your bus factor reduction efforts and the strength of your insurance prior to the emergency. It is highly possible though that the new support team members are actually coasting up until the disaster so you didn't really have the insurance you thought you were paying for.