Y2K all over again
On the day we leave, deal or no deal the Earth will keep turning and nothing too serious will happen. Life will go on, things will get done in pretty much the same way, and problems will be quickly solved or temporary workarounds agreed pragmatically between the interested parties even if the bureaucracy is not yet in place. The only "insurmountable" issues will be those deliberately engineered by people with an axe to grind in order to prove a point.
I am totally opposed to leaving the EU for several reasons, but that doesn't mean that I think that leaving will cause immediate disaster.