All that the story of the goldsmith and the five-percent-per annum loan proves is that the goldsmith was stupid to agree to pay that much interest.
Well, yes. Often it is our stupidity that makes us fall victims of usury. At other times it is the last resort.
It is by no means lawful to induce a man to lend under a condition of usury: yet it is lawful to borrow for usury from a man who is ready to do so and is a usurer by profession; provided the borrower have a good end in view, such as the relief of his own or another's need. Thus too it is lawful for a man who has fallen among thieves to point out his property to them (which they sin in taking) in order to save his lifeArticle 4. Whether it is lawful to borrow money under a condition of usury?