Rust compiler is famously backward compatible all the way to 1.0 (after almost a decade of zero guarantees). My codes targeting early 1.x releases still work in the modern Rust, though the convention may significantly differ from the modern Rust. Your point might be true for some other languages but there is a good reason to refute that for Rust.

Ah, and many F77 libraries are still in active use. Nowadays they are used via C or Python wrapper, and I guess you said "using it as it is" to disregard that, but we were talking about what original maintainers are expected to do and the original F77 authors have nothing to do in order to stay relevant here.