OBP and often would see 8-10 pitches an at bat. Schwarber didn't come close to duplicating this. I think the Cubs greatly underestimated how important he was to the offense. Fowler has also had a drop off, but his OBP is still .330 which is a lot better than the other guys the Cubs tried at leadoff (except for the 10-12 game Rizzo experiment.
The starting pitching has had a huge drop off. Maybe the innings from the last 2 years have caught up with them. Lester, Arietta and Hendricks didn't miss many starts in 2015 and 2016 and pitched another 16 or 17 games in the playoffs. Maybe they have tired arms. I don't know.
The fielding has been suspect too. They just are not playing good baseball.
Unless something changes the Cubs are going to miss the playoffs. Even if they make the playoffs, they aren't playing well enough to beat anyone in a series.