on the other hand, that makes Rust's learning curve way way more gradual than C++. An intermediate programmer with 3 months of Rust experience can reliably and confidently cobble up the libraries together to contribute to the project. Whereas such programmer will be a heavy burden (for other seasoned devs to guide/review/feedback) in a C++ project.

Also, building low-level libraries that work correctly, efficiently, relatively-safely in C++ is NOT a simple feat, and requires years of experience (on API design). With unsafe Rust you can just fuzz/brute force out the unsoundness with the safe API.