Rust's raison d'être is to improve the confidence of the security-critical parts of your system. You don't need to rewrite all 30M lines of code to benefit from it, you just need to identify the 1% of your codebase with the greatest attack surface (e.g. any internet-facing string parser), cordon that part of the codebase off with a C ABI, and then convert that part to Rust. This is similar to how Firefox incorporates bits of Rust into its own C++ codebase over time (e.g. for parsing URLs).