Because if DA leaves he will be leaving because he is fired, not because he is retiring after a long successful head coaching tenure which DA was a big part of.
If DA gets fired it will because they win fewer than 8 games and if that happens it will mean that the offense was not good which means that the entire staff is going to get fired in DA gets fired. This isn't hard. When the head coach gets fired so does the OC and I don't recall a single time that a member of a fired staff got the job as the HC. Can you name one?