[batshit_beaver]

In my experience it's virtually impossible to reach the same level of familiarity with a codebase as the people who originally wrote it. This is not only due to years of accumulated complexity and debt, but also the fact that code simply does not capture business intent completely, let alone correctly or with full context.

This is why projects tend to be put on maintenance mode or rewritten (piecemeal or from scratch) once their original owners are gone. This can be, but isn't always, expensive for the business' owners.