A Rust rewrite would have an easy way to expose an API, something they're still debating how to do and deferring to 7.1.

But the team has already choose. They explained their reasoning and IMO it makes sense: they didn't want a rewrite, they wanted a bug-for-bug file-by-file translation. With a borrow checker and no GC, Rust sometimes forces you to structure things differently (especially in a compiler that usually has a lot of circular structures), so it was not worth it.