It is hard when you mix them in one codebase and need bindings and wrappers for interoperability. This always introduces additional work and maintenance burden. It's always a tradeoff and for most projects not worth the effort. Tech corporations that do this regularly have dedicated teams to deal with boilerplate bullshit and tooling issues, so that regular devs can just code with minimal friction. Rust-in-Linux community decided to take it upon themselves, but I'm not sure if they can keep it up for years and decades in the future.

Though gradually getting of C is still a good idea. Millions of lines of C code is a nightmare codebase.