Successful products are like the ship of theseus, they can go through the hands of thousands of engineers, the important part is having people actively working on it overlapping with more experience people. You don't literally expect the original author to be there forever, butyou do expect people to come onto the project and learn about it whilst the more experienced people are still there. If literally everyone working on a project is gone, the project is probably unmanageable at that point anyway, there's no point starting CPR, the project is dead.
