It's not done for economy, it's done because they are coaching Belichick's offense and defense and not their own and he prefers to work with personnel he's taught and evaluated himself within his system. They'd have paid RAC and Charlie to stay. They let them go because they knew they were itching for their shot (to fail I might add) and BB doesn't like to deny anyone that once they've earned it because he long worked for a guy who did.
Bill has total control and final say in all football matters here. Staff, player selection, scheme, game plans, play calling, USE OF PREVENT DEFENSE. If you want to criticize those it's fine. Just don't try and play the hiding behind the coordinators card 'cause it won't work here. If there are deficiencies anywhere within this organization they are Bill's responsibility.