Two things about that. One is that before worrying about the lead we should finish the discussion of naming in the body of the article. The place to do that would be the "Name change" section? Currently we have a lot of sources speculating on what would replace Metro. Now, in retrospect, we know. Second, we should keep the design language separate from what is produced with the design language (the UI). IMO "Metro design language" is fine as the article title. It's the historical name that everyone knew. If anything,  "Microsoft design language" would seem to be the strongest candidate to replace it. But this is different from all of the different official and unofficial names for the UI that users see, which tend to be tied to the products  Zune UI, Windows Store apps, etc. –