The rationale is that right now, Google has a desktop-first index. So Google doesn’t really index your m-dot; they just annotate the m-dot URLs, but there is no true indexing of your m-dot content. So if you did a migration from m-dot to your main www now, Google doesn’t have to index anything, it just updates those mobile annotations to say the main website is mobile-friendly because it is responsive. If you wait to do this after the mobile-friendly index rollout, then Google will fully index your m-dot content and URLs. Then the migration will take longer, because Google is not just updating URLs but also the content and signals within your pages. In short, it might make sense for you to push up your m-dot migration plans now versus waiting. Get the full story at Search Engine Land Read also "Google’s mobile-first index likely not coming until 2018 at earliest"