Fortran still plays a big role in scientific computing. I am working on climate models, which need to be run on several different supercomputers with different compiler stacks and architectures. Biggest upside of switching to C++ is to use libraries like Kokkos (which aggressively incorporates new C++ standards) to take advantage of the heterogeneous architectures in a portable fashion. And the code will be used / developed by a lot of people, so I guess C++ will win out over just C.
We’ve talked about using transpilers and some of our custom tools and we will probably use them for certain modules. But, one of the exciting parts of the re-write is to have an opportunity to reconsider the basic code structure.