By the way, having irreplaceable people is bad, but not because they are a threat. The issue is the "hit by the bus" problem (will your organization survive if the irreplaceable person is hit by a bus tonight?) and the solution is to train more people to the same skill/competency level.

Absolutely. My proposed solution was to reassign the 'irreplaceable' to training potential back-ups, and only firing those that refuse to do so. (A lot of people found this idea problematic, because it could lead to them creating unofficial coalitions and favor networks that did not respect the command structure).