You'd have to think that Bill is smart enough to know he's not getting it done, Tedi Bruschi alluded to that fact earlier this week.
He should 100% be allowed to resign/retire under his own authority. Any talk of firing him mid-season is ludicrous, done by people with an axe to grind, and should be given zero credibility. Kraft knows that Bill Belichick and Tom Brady made the Kraft family millions of dollars and changed the value of the franchise forever, and he's not about to damage that by making a knee jerk reaction because some talking head or internet keyboard cowboy has his panties in a bunch.
The scenario of Bill being too stubborn and giving Kraft no choice is not going to happen, much to the chagrin of aforementioned haters. Bill will almost certainly bow out, which again was alluded to by Tedi.
Fire Bill Now folks will have to calm themselves. He will be retired, and he will be given an elegant send off. He's not gonna end up like Ned Stark with his head on a spike. (see what I did there?)