Dortmund threw away the league last season, all they had to do was win their last game.City finished on 89 points last year. Even allowing for the last game, the max is still 92 points. That's a hard total, but it's not the massive totals they put up going head to head most years v Liverpool.
LAst year was a great opportunity for Arsenal to beat them. And they threw it away. They had one game a week to prepare for and City had a full schedule. They took full advantage of City's mediocre form in the middle of the season, but they couldn't sustain it.
City have an even bigger schedule this year with the club world cup. It could be a good opportunity for Arsenal or Liverpool again, if the constant fixtures cause injuries, tiredness etc.
Arsenal's challenge was already over 3 or 4 games out. Yes, all Arsenal had to do was keep winning with about ten games to go, but they didn't have the strength in depth of City's squad.