But this is not a good analogy. Rust is being talked about explicitly as "better C++", with a huge and vocal group of online people going around and saying that the world will end if we don't rewrite everything in Rust, or that no new C++ project should be started today with Rust being available.

Very few people, if any, put R against Fortran in this way, so the fact that Fortran libraries weren't rewritten in R doesn't contradict anyone's expectations.