The negative is that time is a finite and limited resource. If a junior person spends their time learning about Fortran or some other esoteric thing while their

The negative is that time is a finite and limited resource. If a junior person spends their time learning about Fortran or some other esoteric thing while their peers learn skills or languages that are in demand, the junior person that learned Fortran will be at a disadvantage. Of course, if you're doing it just for fun, then I'd say there's no negative. But from a career perspective, you'd be better off learning more relevant skills.