People leave because they get bored, so they would switch teams from time to time, give new responsibilities and try to keep everyone motivated. Didn't always work, but they managed to keep some top guys for very long...
I personally believe that "you will be given more challenging work" should be the reason to be given for documenting stuff, rather than "I know you will quit one day".
Not wanting to be interrupted, to know how things work, when you are working on your next interesting stuff, is reason enough to make things better maintainable.
I personally held that belief and worked through such challenges for over 8 years, until I got bored switching teams and creating new stuff for my employer. I now do the "creating new stuff" for myself at my startup.
If the talent is trying to make itself irreplaceable - like say, only one person knows how something works and they do not like documenting it or training others. It shows insecurity of the person who does not want to lose the job, which is bad for the organization as well as for the person.
Bad for the organization because, if the person either wants to quit or cannot turn up for work, a crucial piece of work cannot be completed.
Bad for the person because, though the person might think he/she is crucial for something work and cannot be fired, they are making themselves tied up to the work and hence cannot be promoted to better opportunities.
A talent should learn and grow. And then to teach others and make them grow. And keep repeating the process. If there is not much you can learn to grow, then quit and join some place where you can.
People leave because they get bored, so they would switch teams from time to time, give new responsibilities and try to keep everyone motivated. Didn't always work, but they managed to keep some top guys for very long...