Start with a world without rhetorical questions. All questions are interpreted as being genuinely inquisitive. You are considering doing X and someone comes up to you and says “Why on Earth would you want to do X?”
Now two things happen. First, since all questions are genuinely inquisitive, you take his question literally and you start thinking of an answer. Why indeed do you want to do X? The second thing that happens is that, again because the question is genuine, you learn that it’s not obvious to your inquisitor that X is the right thing to do.
That’s compelling information if you happened to be wondering whether X is in fact the right thing to do. And no matter how successful you are at coming up with an answer as to why on Earth you want to do X, that information will make you at least slightly less sanguine about doing X.
And that is why you can’t have a world without rhetorical questions. Because in a world without rhetorical questions, questions are effective rhetoric. Indeed a world without rhetorical questions maximizes the rhetorical value of a question.
In a world where all questions are interpreted as genuine queries, someone who is not genuinely inquisitive but in fact has an agenda most effectively erodes your confidence in X by saying “Why on Earth would you want to do X?” And so questions spontaneously become rhetorical devices.
As these devices are used more and more, the questions are taken less and less literally. In equilibrium the incentive to convert rhetorical arguments into questions continues right up until the point where questions have no rhetorical value over and above just saying outright “X sucks.”
That doesn’t mean that rhetorical questions die away. They must continue to be used just frequently enough so that their value is just degraded enough so that nobody has any (strict) incentive to use them any more than that.