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.
It won't be impossible, but it will be challenging. I think the upcoming season will be very difficult since Chelsea and Liverpool will have learned from their mistakes and tried to make them right before the season starts, which means they are also contending for the title. When challenging for the title, getting off to a strong start is crucial, but over the long run, will they be able to maintain consistency when player injuries and fatigue set in? If so, will their squad have enough depth to withstand all of this? That's what happened to Arsenal at the close of the previous season; they were helpless to withstand the toll that injuries took.
I'd like to see a team other than Manchester City win the title.