See this as a compromise. A influx of new players is not a bad thing, even if the hooks they were brought in were very handholdy. At least some of the new additions teach players how to play the game, without them having to through a tiresome tutorial or the fundamental design of the game being changed. They are actually playing the game as it was designed with other players, albeit with much more feedback as to what is happening in-game. Some will undoubtedly stay in this safe social playlists, because they like the core mechanics and is unique enough to play casually with their friends. Some people just prefer it not be too competitive, or find that scene or atmosphere a bit daunting. A no-pressure zone is actually welcoming to new players.
Some will seek a better challenge with ranked matches. Lessons learned in the social playlist will help them understand the fundamentals as they were designed, the training wheels are now going off. Play the game as people have been playing it for ages, with map knowledge being key and ranking being the driving force to get better. The plus side of this method is that long time players can directly move to ranked, because they know the game and dont have to be bogged down by all the newness. It worked well for Halo 2 and Halo 3, and both those games attracted all kinds of audiences without the games feeling that they were built to bring in only one kind of audience. Halo 4 kind of threw everybody in the melting pot, and it didnt work; no one core group was being satisfied.