Brexit is really a political issue. There is no logical reason for it, from an economic or business or commercial point of view. So for that reason the negotiators ( particularly the British negotiators) will have to judge the political impact of another delay. Like a plane lumbering down the runway, with a broken engine, there is a point of no return. You have to take off and deal with the problems in the air. I get the feeling that Brexit is too far down the political runway and they will have to go for it this time. The calculation might be that they have to get on with it, and then deal with the consequences as they occur.
That is an exceptionally high risk strategy and would cause a lot of collateral damage! If you cannot feed or cloth your people in January there will be hell to pay and fine, you can blame it on the EU, the French fishermen, the Irish the lot but it still doesn't sort your problems. What are your options the... go to war!