National labs should be able to attract people who can learn Fortran well enough to maintain those codes. After all the hard part is the math, not the programming.
You'd be surprised. People who aren't particularly interested in programming qua programming don't typically have strongly held opinions about it. Using the same programming language as the group you're working in is usually the main consideration. Speaking from experience as someone who has worked in an applied math group where the predominantly used language was Fortran.