Even if you envision everyone migrating from C to Rust on principle (despite the risks inherent to such a large project), I really can't fathom existing Java and C# codebases going anywhere.

For that matter, the COBOL codebases are still out there.

> Even Python interpreter is in process of using Rust as the main language if I am not wrong.

They are beginning to take steps towards making it possible to include some Rust code. Nowhere near "using Rust as the main language". (Unless you're looking at alternate implementations, like the aptly named https://github.com/RustPython/RustPython .)