FORTRAN codes persist today because (1) the old school memory model of FORTRAN is fast, and (2) it is so easy to write numeric codes that do the wrong thing with rounding and numerical instability. There's a reason why Foreman Acton wrote a book titled Numerical methods that (usually) work.

https://www.amazon.com/Numerical-Methods-that-Work-Spectrum/...

Code something up in C, Haskell, oCAML or CUDA and you miss out on the 40+ years of experience people have had with a FORTRAN code from the 1970s.