1. Imports from EU are higher than pre-Brexit and higher than exports. So it's the deficit is also higher than before. 2. The EU does not have the UK as their biggest (40%+) market. Therefore the smaller player is the UK. If all trade stops - yes both sides lose. One collapses economically overnight. 3. The EU can either absorb the cost on its internal market (and sell goods internally for a smaller margin) or export to other players as there are existing trade agreements (again with a smaller margin). The UK does not have access to the same deals. 4. The EU market has a much higher GDP. Losing 1 trading partner even of the of Britain would not take down the Union.