Julia core devs are aware that Julia does not exist in a vacuum. It is easy to wrap existing C, C++, Fortran, and Python libraries for use in Julia. Julia has such good performance and flexibility that it will not surprise me if the most important libraries get rewritten in Julia. Most people just glue libraries together most of the time (and that's perfectly fine!), but language improvements will mostly be seen by the people who write the libraries themselves.
See:
- https://docs.julialang.org/en/latest/manual/calling-c-and-fo...