They probably should rewrite it in Rust, but also, I think the Bun guy wanted to rewrite it in Rust because he thinks Rust is a better choice for Bun, independent of Bun being used by Claude Code.

This speaks for the benefits of JavaScript runtimes like Bun and Node.js. JavaScript is a dynamically typed language which makes it a good choice for explorative programming. It is a Lisp in sheeps' clothing.

Programs which need more stability and performance than exploration are better done in statically typed languages like Rust. Therefore Bun is a platform for JavaScript programming, but itself is written in Zig or Rust.