It is not about fashion, it is about not being ad-hoc. For small scale problem where most of the variables are well understood, this kind of approaches work be

It is not about fashion, it is about not being ad-hoc. For small scale problem where most of the variables are well understood, this kind of approaches work beautifully. Big problem are better tackled by a more generic approach (maybe with some ad-hoc adaptations, such as mixed approach between expert systems and statistical algorithms, feature engineering, etc.) because these approaches will be more resilient to an exposure to the real world, and the manpower invested in them is useful in more than one problem domain. To address your last point, there is an extensive body of work on data-scarce environments. I've even seen a talk about applying reinforcement learning to endangered species preservation, where you only get a single digit number of interaction with the system !