I'm sticking around for personal/defensive reasons (it's stable and recession-proof, and I have a one-month old so moving would be extra stressful for the family), but I doubt I'll stick around for more than another year or two. My turn for that revolving door is coming up
What we did was that we made our internal docs and agreed that whenever we were blocked by something, once we learned it, we would document it. It was a mixed success though because we never had time to work on the docs properly, and the veterans didn't touch it, so I'm curious if there is a better approach.
Doesn't help that some of the engine code dates back to the late 80s, was originally written on a mainframe for a compiler where variable names had character limits, so even if you know the surrounding conceptual terminology it's functionally indecipherable unless you have a lot of experience with it. It's hard to tell what it's doing or why, or even if it's relevant to the problem you're trying to debug.
What we really need is something like a dedicated sprint where we stop the presses outside of critical maintenance and just write documentation, free up the veterans to write down their knowledge. But that would require the company to actually invest in what is otherwise seen (wrongfully IMO) as a cost-center. :P
And honestly if we had that kind of money there's other software redundancies/inefficiencies that need to be resolved first, that would lower the documentation requirements in the first place.
In the office the culture for accessing these guys was: Ping with slack DM/skype, if they don't answer in 15 minutes walk over to their desk and bend their ear, unless they were clearly working something more important at the moment. Sometimes there would even be short lines of people outside of their cube waiting for attention.
There was an organic interrupt-driven workflow for these experts because they were in so much demand (with occasional pauses in access if they were working on something time-critical). That whole workflow collapsed under WFH, and as these veterans were used to operating on their own/managing their own time roughly half of them failed to be readily available online. This led to issues for people like myself where we needed some piece of their knowledge but just got radio silence for hours, sometimes days. So you pick up some other work to fill in the gap while you wait for a response, but that leads to inefficiencies of its own once they actually do get back to you. Once I had a cascade of about 6 stories going in parallel because I kept picking up alternate work to fill gaps, only to get to a point in THAT work where I needed some tribal knowledge, so I pinged the appropriate SME, waited several hours, picked up another task in the meantime... etc
And leadership lacks any carrots/sticks to make these guys accessible aside from asking nicely. They're largely where they want to be for their careers, value their time, and have all the truly vital knowledge so they can't really be punished lest they leave the program.
Sadly I suspect my company's organization is closer to the average than yours, so lots of other companies fall into this boat. It's not always a matter of discipline :)