As long as Marrone is HC, they'll never hire another OC. First, it will signal that Marrone, being a former OC himself, was a poor judge in identifying the qualities it takes to be a good OC. Second, it will show that Marrone has the inability to teach an offensive-minded coach on how to be a good OC. Lastly, Marrone will likely take over play calling before he fires Hackett. And if this fails, then I doubt Marrone will be around to hire another OC.
I completely agree on the firing of the ST coach though. In fact, that would be the admirable thing to do.