The funny thing about legacy code in scientific programming is that it is not so much of a "bad smell" as it is in commercial work.
It is hard to write numerical code that consistently works correctly in the face of rounding errors and the like, and it is hard to know if you are getting the right answers in many cases. It makes sense to use the old code (which performs well) rather than write something new and take the risk of screwing it up in some subtle way.
It is not hard at all to access FORTRAN data structures from C, Python, Java or whatever your favorite "modern" language is.
It is hard to write numerical code that consistently works correctly in the face of rounding errors and the like, and it is hard to know if you are getting the right answers in many cases. It makes sense to use the old code (which performs well) rather than write something new and take the risk of screwing it up in some subtle way.
It is not hard at all to access FORTRAN data structures from C, Python, Java or whatever your favorite "modern" language is.