That might work in the short term, but it's not a good negotiating tactic. You need to build trust. If the EU had a deal on the table then engaging in that builds trust even if its not yet the deal you want. If you go in saying that you'll need to increase tariffs because the current system isn't working unless a deal is made - then fine. If you blow up the economy first and abandon the rules and order you put in place in order to try and strong arm an agreement then even if you get that agreement, you're the mob, the bully. Everyone will be working out how to get away from you, they'll have no trust in your agreements.
He's already done the crazy thing. The negotiations are already compromised.