As you might expect, promoting the best works well if future competence is correlated to past competence. Promoting the worst works best if future competence is uncorrelated with past competence. Promoting at random works no matter what the correlation really is, and promoting the best or worst can be made to produce the same overall outcome no matter what the future competence rule is.
I find this rather disquieting when you try and map this into real organizations. Could you really do promotions on a random basis ?
However, it is rarely the case that things are so easy to measure. So-called "soft skills" such as communication and the ability (or willingness) to manage staff will often give organisations the additional leeway to promote a person who probably isn't actually very good at what they do. Sometimes (often?) this decision will be made because political alliances have been made. Other times it may be to fill a racial or sexual quota.
These additional factors probably introduce a good deal of randomness into the mix when measured over time and averaged over all appointments.