Also the division of roles is far more specialized than say basketball. Only the QB might have a near/full picture.

Football team coaching staff are almost like another team. Position coaches usually are ex players of that position but as you move up the chain it gets more into strategic management and thus by the HC level it's more of a manager role with architects underneath for offense and defense so less position technical knowledge more game knowledge.

> I can't think of any sport where players are expected to become coaches.

In amateur sports it's common. E.g. in rowing, cycling, fencing, cross country skiing, etc., people are generally expected to coach at some point. Albeit usually while they're doing the sport, and not necessarily as their primary careers.

Football/soccer? Of course there are more players than coaches so they can't all become coaches, but I would say star players are expected to become coaches, and lots of them do.

Oh, I agree, due to it being a pyramid only a fraction could ever become head coaches, so that wording was a stand in for that effect.