When I was working for LROC, most of the planetary geologists used IDL (Fortran-ish in many ways) but there was still plenty of actual FORTRAN floating around. Oh, look, in the last couple of years this legend of photometry worked up this new method and here's the associated Fortran code. That kind of thing.
I did actually rewrite both IDL and Fortran, but it was always smallish, single-purpose programs or functions.
Note that the SPICE Toolkit[1] is still written in Fortran today, translated to C using f2c, and the C version used as the base for other language support. The Fortran part is unlikely to ever go away, since support for processing past missions is crucially important and all that processing code was written in Fortran. Also, talk about your stable, backward compatible APIs...