This is just common sense. The EU is unlikely to be totally negative during negotiations as the UK is an important trading partner for many EU countries as they are for it. Europe will, however, have to ensure that Britain does not gain at the expense of any of the EU members so it will be impossible for them to be better off in trade terms with Europe (the UK's main trading partners) once Brexit actually happens. It's a bit like someone trying to negotiate themselves out of a commitment to a gym. They must have thought that the benefits of membership were worth it when they first signed up and now they have changed their mind it is in the interest of both parties that they are allowed to break their contract. It would be unthinkable, however, if the gym allowed them to leave but allowed them to carry on taking advantage of the facilities on more preferable terms than the committed members.
As I wrote before, Britain will leave the EU in a worse economic position that it currently enjoys. Only time will tell if it can benefit from being an independent sovereign nation and use its unique situation to rise to the dizzy heights that have been forecast and promised by the Leave team. My major concern is that not one of the aggressive Leave disciples who post on these forums has once explained how this miracle is going to happen.