Why? You are assuming that the 27 are going to be united on their demands, some countries would accept any deal likewise to prevent the loss of UK trade. Poland just for example will certainly not care too much about the wishes of the Germans as opposed to keeping its citizens ability to work here. In fact it is probably the Germans and the bigger countries who stand to lose most so the lobbying power there will be huge to keep things sweet.
It isn't as simple as the EU is this all powerful entity who we must bow down to because they are so mighty, just for a start they will collectively lose far more than we will that is almost completely certain. Just for a start they will lose the hundreds of billions we send every year and that is a huge problem given the potential for change to future spending. We on the other hand will regain all of this money in the worst possible scenario which could be used to at least cushion the change.
In my opinion we should contribute but contribute less to keep free trade free whilst retaining what we want, it is either that or they lose hundreds of billions along with the free trade that we AND they enjoy.
We are in a good position but it will take a long time to sort, I really wouldn't write us off quite so easily because we are a massive country by weight, it is finding the right balance and giving THEM everything they want by accepting any offer would be ridiculous.
The biggest threat in all of this is not the reality but the politics of the scenario, that is the EU will potentially put power ahead of common sense and that would be a disaster for everyone. Leaving common sense off the table will result in a long drawn out exit where uncertainty prevails and basically no-one gets a good deal.