What I mean is, if you're going to rewrite it in Rust, why rewrite Babel rather than leaning on the existing ecosystem? I know they're not actually rewriting Babel, just reusing the semantic layout of its AST, but it's feeling a bit like the MediaWiki parser situation to me (roughly "if we started from scratch today, we wouldn't choose to have it this way, but we started a different way before, and it's been a difficult path to get to where we want to be"). Maybe that's a fairly remote analogy but it feels similar.