Oh gosh, I don't think I can recall a specific source. I've listened to many interviews with the TypeScript team and in Q&A they're often asked about their "biggest regrets". Early on TypeScript's adoption was far from a sure thing. After convincing Microsoft their biggest threat was Angular's own AtScript (and maybe even Flow). TypeScript was extremely beholden to whatever Microsoft or Angular devs wanted to be added to the language in order for them to agree to push TypeScript as the future