As other comments have noted, the point is that the EU will not want to give the UK a good deal on leaving, because the EU does not want to give any encouragement to the other countries which want to leave the EU (some of which want to leave more strongly than the UK do - apparently). Secondly, why would the UK end up with a Norway or Switzerland deal, when the UK is the 5th largest economy in the world? This is unpr…
This is pretty reckless risk management on Britain's part: to place so much weight on a question of just how a future, unclear negotiation, subject to who knows what bureaucracy and other countries' own interests, will turn out. It's very vague to just say "well we're trading a lot, so surely we're very important" without any supporting evidence or precedent: it makes all sorts of unjustified assumptions.