What should they move to? Fortran90 has vector and matrix notation build-in so it's just like typed, faster matlab. More safety, more speed, what's not to like?
Move to C? You lose the vector notation, and gain all the pointer mess, so not very lucrative unless you were a C-programmer to begin with.
Move to python? Sure, the language is nice, but it's not faster than matlab, so why bother. Unless you really appreciate the extra niceness. (I do)
Move to C++? Again, no build-in vector notation, so not very lucrative. Sure, you can use libraries that overload operators to get the vector notation, but the learning curve can be steep, and after all the extra learning, you just have what you had with fortran90 and matlab to begin with. Of course, if you already were a competent C++ programmer, then you may like to do everything with C++. But scientist may not have that much extra time to train themselves even half-fluent in C++.
Move to some other language? Which? Java? Scala? Rust? Haskell? Lisp?