How to handle a “Hero” employee that keeps valuable knowledge to himself?

Problem

In my previous company, there was a gentleman who worked for me who oversaw our production servers, that for this story I'll call Brian. Before I was his boss, he was the only person who understood the configuration of our production servers. Brian claimed to spend countless hours every day keeping them up. Whenever there was a server issue, Brian would go to his desk and emerge victorious, having fixed the issue. What he did was unclear, and when asked he would give a dismissive answer, with no details, just acronyms, and technical jargon. As a result, he could never go on vacation, leave early, or have a night off, because if there was a server issue, we needed Brian. He loved the adoration of the owners of the company and his colleagues. However, where the owners saw high value, I saw high risk.

Actions taken

I addressed this by appealing to his emotion and logic. First, I spent some time talking to him and learning who he was, trying to figure out why his situation was what it was. I made sure to thank him personally and publicly for the times he saved the day. This worked into conversations about my concern about his work life balance and burnout. Truthfully, it was horrible - Brian was on call every day. Then, after a few "production salvation" events, the conversation shifted from emotions into logic: "What if you get burnt out and quit? Or, you get hit by a bus? How will we protect the production environment?" Additionally, I began engaging with him during these events, illustrating my system engineering knowledge. Next, I assigned a junior engineer to be his student. Brian became his technical mentor, teaching him all he knew about our environment, which Brian loved.

Lessons Learned

Asking the right questions exposed the fact that he wasn't the only who could do what he was doing. I took notes and even took the lead the next time we had an issue. The end result was more people with critical knowledge. Brian could take days off if he wanted to, and the company's exposure was mitigated.