Being unaffected by fashion trends is extremely important. In almost everything people do, from choosing a problem to solve to buying art, there are always some unfashionable options that are not only better than fashionable ones but also cheaper because they're unfashionable. When buying goods, cheapness is a tangible price. When choosing a problem to solve, cheapness means they have higher expected value, mainly due to less competition. Now, whenever I enter a new field, my first thought is, "What are some undervalued options because they're unfashionable?" The result is always a multitude of such options. This is a great way to quickly get to know a field.