i think of a prototype as a question with buttons.
it can ask whether a workflow makes sense, whether the language is clear, or whether a person can recover when something goes wrong. if it only proves that i can make a screen move, it has answered a much smaller question than i need.
the useful prototype is narrow enough to change quickly and real enough to expose a decision. i want to put it in front of someone, watch where their expectation differs from mine, and then change the model.
the goal is not to defend the prototype. the goal is to learn what the product should become.