This is a terrible way to maintain a system. You never know when the guru is going to die or quit; you have to have a way of getting other people to maintain the system. This is why humans invented written language: to transmit information to each other and to subsequent generations.

However, writing useful documentation isn't trivial, and is best done with a technical writer, and takes time away from development. If management isn't willing to invest the time and resources necessary to produce useful documentation, then they're going to have problems in the future as the gurus leave.