For someone starting out, there's such a thing as opportunity loss. What language would that person had learned if they had not spent that time learning Fortran? Would that language had been more marketable in their next job?
Further, what you used your last job matters. Particularly if that job is the _only_ real job since you got out of school. If you just learned Fortran for fun or for a job in a long list of jobs and your github account is littered with good work in multiple languages, Fortran can indicate flexibility, diversity, and a a broad skill set.
If all I see is one job with Fortran for 2 or 3 years after school I might hesitate to hire you for a <insert language here> job at anything but entry level.
edit
Just a note, I think Fortran is a fine language and anyone whose done a decent amount of linear algebra work knows what a big role Fortran plays in analytics.
My main point is that you should care about what your first job(s) are coming out of school because they can shape what the next jobs are going to be. So make sure it's something you want to be doing.
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.
It's not like learning FORTRAN (or C, or Pascal, or COBOL) will make someone unable to program in other more markeatable languages.