I was listening to an English-language reading of the Quran out of curiosity and it very plainly tells you that usury isn't allowed. As far as I'm aware, the Islamic Caliphates phased out interest in the territories they conquered, and the Islamic world only really began allowing interest again in the 19th century as a reaction to European dominance of the world making it inconvenient to ban usury for much longer. If Islam hadn't prohibited usury in the first place for whatever reason, how would the development of the Islamic world and the world in general have changed?