This is why a documentation culture is so very important. Even on the best terms, the average tenure of a tech employee is two years. That's just today's economic reality. As a leader, a project should not be considered 'done' until its documented to such a level that a well-meaning new employee can come in, read the docs, and be mostly up to speed with at least the daily 'care-and-feeding' tasks and the top 5 failure modes.