If a company is dependent on one (or more) people to perform then it is in danger. Processes should be in place that don’t risk the success of the company on the performance of a specific person. If your organization is dependent on a person start taking actions to place the success of the company in reliable processes instead of individual stars.
In many instances the start is as simple as starting to document processes using flowcharts. Another benefit of doing this is that you can then make sensible improvements. It is hard to make reliable improvement hen processes are not documented and instead remain the mysterious realm of individuals within the organization.
Do your outstanding people think their importance is in getting through another day with through hard work and individual expertise. While those qualities are good most important to the success of the organization is improving the system not getting through one day. If those seen as the stars are not improving the system and processes then get them to work doing so. They might miss the feeling that the organization is dependent upon them. But it is more important that the organization not improve. And there contributions will still be worthwhile but the organization will be much stronger.
Performance dependent on specific individuals is not robust and not capable of continuous high quality performance.