With regard to point 1, I think the reasoning is that they wanted precise control over their .d.ts file, since that's the public interface. This I understand,

With regard to point 1, I think the reasoning is that they wanted precise control over their .d.ts file, since that's the public interface. This I understand, though I don't see why switching to JavaScript necessarily helps. Maybe it was tricky to get the TypeScript compiler to accept their hand-written .d.ts while also processing TypeScript? Which sounds more like a tooling problem than a language problem. I'm with you on the rest of the points, though. I don't quite understand it, either. But then I'm not working in that codebase every day. Easy to criticize people from the outside!