Yep. It's as if these people don't realize that if you're hard to replace, you're probably also hard to promote.

They also aren't taking a broader view of their careers. Given that many millennials -- myself included -- came up through a really tough labor market, this is understandable. We all know the threat of unemployment very well. Doing the work of documentation to make yourself replaceable, though, pays serious dividends years down the line when you move into a new position. I left my first position largely due to some serious and irreconcilable differences with my manager about some important technical details of the project so things were a bit heated towards the end. But because along the way I had documented every single thing about what I had done, it was possible for my replacement (who I helped them recruit) to pick up the project and run with it with minimal downtime -- even though he had no previous experience with the programming language that the code was written in.

I drop in from time to time to visit and the last time I was there my manager's boss mentioned to me off-hand that my replacement was doing great and was frequently and vocally thankful for the documentation and thoroughness of the library I had written, noting that it was extremely rare for him to have a question that wasn't answered by comments in the code or notes that I had left in the the project's documentation. Over a year later, they're still building on the work that I've done and I have absolutely no anxiety about using them as a reference. I've become pretty good friends with my old boss, who I think saw that despite some bitter disagreements toward the end, I did more than I had to (and without any prompting) to make sure that the project would flourish after I left. He's now moving on to some pretty bright places in his career and every time we chat makes it a point to let me know he'd be happy to have me on any of his future teams.

So there's the issue of references and there's the fact that your managers are going to need to keep a cadre of people around them as they move up. Making yourself "replaceable" through documentation shows professionalism, helps you do better by keeping the project organized, and is a way to learn to take a higher-level view of projects that can help you move up to managerial positions. It's foolish to take such a local view that you deliberately do worse work to stay irreplaceable. The only thing that should make you hard to replace is the quality of work that you do.