Monsieur,
Granted if you have two equally qualified candidates, not only technically but also in leadership terms, for the responsibility of commanding a crew, then I agree you're right. But in reality, how often does one truly get that situation? Your case is also based on the premise that those who aren't suitable for command, but are perfectly good FOs, get dropped by the company so that only command-capable people are left when their opportunity for promotion comes about. Is that the case or are all candidates believed to be command capable by the time their promotion slot comes about? Is it done on merit or merely time served?