Exactly this - no benefits huge cost and disruption. Can you imagine any organisation signing off on that as al business case;
Strategic Case (the case for change, strategic fit) - alienation of neighbouring countries, crippling to our economy, trade and jobs, vague notions about trade alliances outside of EU. NI thrown into turmoil. But we will have Sovereignty (whatever that is).
Economic Case (public value): Plunging pound and unknown impacts of no trade deal being struck with EU. Likelihood of import/export channels for goods and people being unavailable due to lack of agreements and reliance on non existent technology. Job losses, impact of public services and infrastructure. So no benefits. But we will have Sovereignty (whatever that is).
Management Case (Achievable, can be successfully delivered): not written, no idea how to deliver it or who could deliver it.
Financial Case (Affordability, within budget): Huge and ill defined costs to government and businesses both in UK and for our potential trading partners. Unaffordable. Potential negative impact for years to come.
Commercial Case (Supply side capacity, a potential deal) not written, no real precedent to follow, no desk certain and any deal struck likely to be punitive and bring many disadvantages