Around every top performer, you need to watch out for a "zone of incompetence" - an area where other people can relax just a bit, think a little less hard, or focus on other issues - trusting that the top performer will anticipate and correct any problems that come up. The high achiever will always make sure the results turn out just fine.
Of course, this is not always the case - some top performers encourage everyone around them to raise their standards, but I find this is less common in practice - it usually takes two or three top performers to "raise the bar" for an entire work team. Hey, it's human nature - we all have too much to do. If you work with a superstar, it's just too easy to trust the top performer and go focus your attention on other problems.
When you manage a top performer, often you simply don't realize just how much they did, or how many problems they overcame ... until they are gone. You can never be entirely sure if they were "carrying" some other people on their coattails, or were just so talented that other people felt like they were "held back" and would cheerfully step up if given the opportunity. When a top performer leaves, sometimes it creates room for other people to shine, but other times, the whole team collapses.
So when you are planning to promote a top person, or thinking about succession planning - pay keen attention to the "zone" around them. Their supervisors, peers and subordinates just might be a bit too relaxed and trusting that things will "all work out." You would be wise to strengthen the zone all around a top person before you consider replacing them, because there is no tougher job than coming in to fill the shoes of a superstar ... and no bigger hiring risk for you.