I think that even after someone has had a coordinator position, the odds are still against them succeeding as a head coach in the NFL.
Not my argument. Most head coaches have been coordinators, and most head coaches fail. My argument is that having someone serve as a coordinator, even briefly, can screen out people who are utterly unsuited for management and responsibility within the organizational structure. Based on what I've seen of Brian Flores, I suspect he would have absolutely screened himself out if he had actually been responsible for anything before being called up by his boy Grier.