The Bun rewrite is a long-term investment; the idea is that you initially compromise on idiomatic Rust for the sake of being able to keep shipping, but then the codebase becomes more idiomatically Rust-like over time. It will probably take another year or so to determine whether this works out.

(Note also that Bun inherently needs to use a lot of unsafe because a large fraction of its internal API surface is interlinked deeply and pervasively with that of JavaScriptCore.)