It's exceedingly rare that you would not be able to figure it out given the source code. The real question is whether you have the time.

I think if a project was that important, the people who needed something patched would be able to get it together even if the original maintainer went missing.

The original analogy to the challenges of knowledge transfer isn't totally applicable. Knowledge transfer is important to organizations because figuring everything out all over again would be costly, not because it would be impossible.