THE IRREPLACEABLES. THE EMPLOYEES TO NEVER LET GO. EVEN IF YOU HAVE TO INVENT A ROLE

Summary from the TLDR Founders newsletter

You will always regret letting one of the truly irreplaceable people on your team leave. This usually happens with scale. As things change, you need larger teams and different processes, and new executives may not think these employees are so irreplaceable. However, they are the ones who truly know how and why everything works and are often the best to help figure out how the next move can work. Top performers are the people you'd rehire into their role - irreplaceables are the ones you would rehire into a role that doesn't exist yet.