Personally, I like to think of Fortran as the language of large-scale numerical PDE and eigenproblem solvers. Most scientific and engineering problems boil down to one of these two.
I personally know and like Fortran in its modern form, at least within its little niche. But the ability to maintain these codes long term is really important and it's tough to see that being viable with the supply of professional-level developers withering to nothing.
Hell my sister (print sales) joined a new firm and she had to spend six weeks in an intense classroom style training program the company ran to get new staff prepared for the nuances of their product.
It seems straightforward to setup a Fortran internal bootcamp for experienced developers at these massive legacy institutions, worth the upfront investment...no?
Of course you could make the case for even more modern languages like Rust etc as well, but the national labs are still on the conservative side when making decisions for their big projects. C++ does hit a nice sweet spot in terms of performance, features, and longevity despite the warts.
If you go into such a bootcamp as a fresh grad, you basically pigeonhole yourself into working in that niche forever. You would have a massive setback when switching jobs compared to taking a bogstandard Java job or whatever.
You're also wrong about the setback and being stuck in that niche. Once you learn the first language, more likely than not you'll have opportunities to continue to grow and develop in the role and learn new things. Most people are pretty adaptable.