I don't think it's really impossible for Man United to take this opportunity this season. As was the case last season, where Arsenal put up quite a fierce resistance to Man City, it's just that they couldn't keep their consistency at the end of the season. If Man United can get off to a good start and they can maintain their consistency, I think Man United can become a serious challenger in the title race. It's not just Man United, I think Arsenal, Liverpool, and Chelsea also still have a chance to win the Premier League trophy.
Obviously Manchester City remain the most favourite to win EPL title this next season too, otherwise Arsenal and Manchester United come second followed by Liverpool in my opinion. I believe Chelsea will improve certainly but it will not fight for the top 4 spots. Newcastle will remain close most likely.
Odds for winning the title is +50% for Manchester City, around 17% for Arsenal, 11% for Liverpool and 7% for Manchester United. I don't know why there is such difference between gunners and ManU, and why Liverpool odds are upper for Liverpool. I think these 3 teams odds should be around 20% at least and 40% for Manchester City.