There are plenty of potential negative developments that can occur if the UK leaves the EU
For example just not having the same access to the EU free trade zone or having additional tariffs on UK goods exported to the EU would be economically detrimental to the UK?
But as they say, our export into the EU is minimal, while our exports to non EU countries are much bigger.
Plus, the EU exports more to us then to other countries in the EU. So if they start raising the tariffs on those, it'll cause issues with the Germans and the rest who rely on us buying from them... Because we'll start looking at other non EU countries to get the stuff from if they cause issues.
You don't need treaties to trade between countries.
Also, other note. The biggest companies in Europe? The majority of them are Swiss. Guess what, the Swiss aren't part of the EU.
But the key thing that I think comes out, would you rather have a great economy, but absolutely no say in your government? Or would you rather have a potentially shaky economy, but some actual democratic government?