Why should we assume that all people who can get a PhD are fit to become profs? Arguably it's good that there's a larger pool of candidates from which the best

Why should we assume that all people who can get a PhD are fit to become profs? Arguably it's good that there's a larger pool of candidates from which the best can go further in academia (in fields where industry is not so attractive or existent). Setting aside whether the right criteria are used for career advancements or not, any meritocratic hierarchy is based on having more candidates on a lower lever compete to get into a smaller but higher level.