Are we at the point yet where we might be able to replace some of these legacy Fortran libs with more modern, legible, performant Rust or C/C++?
It is mostly out of habit. These days there are no differences between languages. Very often they compile to the same machine code. Fortran array syntax is light years ahead of what C++ offers (yes including std::linalg and that's why numpy, julia, matlab picked it up) but lacks everything else as a language including often claimed portability.
It's an example of Chesterton's Fence.
Also with C99 you get many of the convenient aspects of fortran (<tgmath.h> and vardim array function args) and with C11 you also get <complex.h> now.
But you are still missing fortran's ** operator and it's more than just syntactic sugar, it accepts more types.