I don't think there is much argument to keep Hackett, other than not to give Manuel two offenses in two years. We all complained that Chan had keys to a Ferrari in CJ and used him like a Pinto, favoring short timed passes. Hackett's schemes/playcalling/use of Spiller have been worse and we've been unable to run more than a few screens all year. Chan, in retrospect, looks pretty solid.
This offense more often than not is unwatchable.
This D, on the other hand, barring a few big plays a game -- which is fixable -- has been impressive, stats be damned.