"Does anyone else notice how, throughout this technical and complicated international discussion about the future of border security, intelligence sharing and anti-terrorist initiatives that there is zero mention of things such as public safety, effective policing or common defence?"
That's because we've spent several decades with the EU as the vehicle for handling these at the European level and by leaving we exclude ourselves from that. It's up to the Brexiteers to explain how they propose to deal with the consequences - it was they who persuaded a small majority of those who voted to bring those consequences down on us. Or didn't they think it was important?