How can a web designer sell this to existing clients?

Start with the customer behaviour question: do your customers use AI before they buy? If yes, the business needs to be understandable to those AI systems. The Digital Twin is the structured answer to that problem. For web designers, the value is being able to add an AI-readable layer beside the client website without rebuilding the whole business from scratch. In other words, the goal is to give the client a controlled first AI layer instead of leaving the whole topic vague and undefined. In the SupplyOnly.com model, the point is to make this useful rather than decorative: connect the answer to approved business facts, public Q&A, image context and the Own The Conversation fulfilment pathway. That gives a referrer or client a clearer reason to act without pretending that AI visibility is magic or guaranteed.