Trade doesn't work on number of countries but share of world trade of these countries.
The EU has free trade agreements in place with 72 countries representing 75% of global trade. We're losing access to all that, if and when we leave the EU.
It will take years to rebuild it. Nobody is saying it's impossible, we're just saying that the negative impact will be felt instantly and the positive one will take years to materialise, maybe about a generation.
The EU has got its fair share of issues but we know how to navigate them, the other guys, not so much.