Most people will point out the support issue, namely getting knowledgeable people to work on it might be difficult. In the case of the code I have worked on while we may rewrite a working system in a newer language it is more typical to wrap known bullet proof functions in layers to present it in new formats; web/mobile/etc.
Most people who know C can pick up Fortran in a few days. I am not a Fortran guru, but have learned enough to write a lot of code for a few scientific projects in school. I really don't understand the mindset that you "need Fortran programers to write Fortran".
