Although I would prefer to not ever see Fortran code (as I don't know how to call into it easily, C implementations are easier to embed in any language), some numerical methods with simple code but heavy math are just not reimplemented too often.

I believe the real value in Fortran is in these libraries that handle numerical edge cases as well. It would be much easier to move them to a newer language if they would contain unit tests, but they usually don't, so it's just not worth the risk sometimes.