There’s this project management idea called the ‘bus factor’ - the number of people in a project that need to be hit by a bus for the project to stall. For instance, I work on a project where I am the only data scientist. Should I meet this proverbial bus, the project would stop. Thus the project I am on has a bus factor of 1. Note that this is not necessarily true for other members of my project. For example, the project manager, UX designer, or the backend architect could probably be replaced or rotated off the project with minimal disruption.
It’s obvious that higher bus factors are preferable, but sometimes one is simply constrained by the available resources. I suspect that this is the case for a lot of startups (maybe there is only one person on the project in the first place). Being a critical point of failure, I’ve tried to mitigate this through a variety of methods - most of them involving documentation of my processes and outputs. None of these are adequate substitutes for a properly trained replacement, and it would probably take weeks to onboard a person to replace me.