> If even AWS—with their resources and institutional knowledge—can struggle when senior folks leave, what chance do startups have?

My personal crusade is trying to convince anyone who will listen to me that the value of a senior employee in any role isn't in their years of experience. It's in their years of experience with your system. More companies should do whatever it takes to retain employees long term. No matter how good your documentation procedures are, no matter how thorough your knowledge base, no matter how many knowledge transfer sessions you have, your long tenured employees have forgotten they know more knowledge than they will ever be able document.

I have never seen a team lose a (productive) long standing member that still wasn't suffering from that loss years later. We like to pretend that software and especially reusable libraries, components and frameworks make developers interchangeable widgets. Substitutable cogs that can simply be swapped out by having enough documentation. But writing software is converting a mental state into a physical one. And reading code is trying to read someone else's mind (incidentally this is part of why two excellent developers can find themselves unable to work together, they just don't think enough alike). When you lose those senior people, you lose the person who was around for that one outage 8 years ago, and who if they were around for the current outage would have a memory float up from the forgotten depths that cause them to check on the thing no one (yet) suspects.

This isn't to say you shouldn't document things, and that you shouldn't work to spread knowledge around. The "bus factor" is real. People will retire. People will leave for reasons you have no control over. So document everything you can, build good knowledge transfer systems. But always remember that in the end, they will only cover a portion of the knowledge your employees have. And the longer your employees are around, the more knowledge they will have. And unless you are willing to pay your people to document everything they know full time (and I assure you, neither you nor your customers are willing to do that), they will always know more than is documented.

Nor despite my analogies does this apply only to software developers. One team I worked on lost a long term "technical writer" (that wasn't their full time work but it was a role they filled for that team). This person was the best person I've ever known at documenting things. And still the team reeled at losing them. Even with all the documentation they left behind, knowing what was documented, where and how recently and all sorts of metadata that wasn't captured in the documentation itself went with them when they left. Years later their documentation was still useful, but no one was anywhere near their level of encyclopedic knowledge of it all.

To use an analogy, the New York Yankees perform the best when they focus on building up their roster from their farm system. Long tenured players mentor the new ones, and a cohesive team forms that operates as more than the sum of its parts. They perform the worst when success and money take control and they try to hire "all-stars" away from other teams. The intangibles just aren't there, or aren't as strong. A good company is the same. A long term team built from the ground up is worth far more than a similarly "experienced" team made up of outside hires.

Talent exodus is concerning, so don't give your people a reason to leave. You can't stop it all, but you can do a lot to avoid self inflicted wounds.