This assumes that there's a significant difference between top performers and the next tier down, and that it's important to keep 'the best people'. In most businesses there are plenty of other good people who can step up if the best ones leave. Its also rare that someone is 'the best' at everything; people have a range of skills and you'll only be losing the person who's best at one or two of them.

Most people who think companies have a 'best person' aspire to be seen as that person, and haven't really thought about what it would actually mean.