You underestimate the the politics of the situation by concentrating on the economic argument.
Of course, economically both sides will lose. The Uk more than individual countries in the EU. we will lose out to 27 EU countries plus access via the EU to about 50 other countries which have trade deals with the EU,whereas each individual EU country loses out by varying degrees to just the UK
But to the EU, the bigger concern is protecting the EU and what it stands for to them Especially Germany, who gain the most from the Euro currency and the Union itself. We cannot be seen to “ have our cake and eat it” as BJ promised to voters during the referendum.