Assuming that the UK can negotiate the trade deal it wants in the event of exit is naive. This isn't about what's economically sensible (for either the UK or the other EU states). It's about politics and history and emotion.
Should the UK vote to leave, there are many many politicians around Europe who will want to punish the UK (even if it causes some harm to their own economy) - as much as anything as a warning to any other potential waiverers across Europe about the consequences of leaving.
If we decide to leave, this would be the most painful and disruptive divorce imaginable.