Jefferson isn't asking to leave unless they stick with McCarthy (and maybe not even then, he seems like a good guy). And they are less likely to stick with McCarthy if they are embarrassingly bad. If McCarthy was sub par but they won nine games, they'd absolutely stick with McCarthy. Him (and Brosmer) being bad is good!
If we want a new QB, and I think most of us do, the worse these guys play, the better our (and Jefferson's) chances of getting that wish. Short term pain for long term gain. Or at least an attempt at long term gain, which is all we can really ask for.