I disagree with your conclusion that Brexit will cause much harm to the UK economy, but even if this was true, leaving the UK and having this harm is within its citizens rights.
The UK held subsequent elections after the Brexit referendum and the party whose platform is "Brexit" has won these elections.
I think it may be the only way to get a trade deal out of it -- as the EU pretty much feels that they can bully the UK simply because 'they probably won't leave who no deal, so they'll agree to something at some point.'
It may backfire, but it's the only thing they can try to get a deal as the deals before this have been pretty much BS by May.