The slide began largely due to the rapid decline in both the Kaepernick lead passing attack and in the ability to stop the pass. Ultimately the passing attack has clearly remained the largest issue for this team. However, the passing attack power ratings show that the passing game, lead by Blaine Gabbert, actually improved in the latter half of 2015. The passing attack ended the year at almost league average level.
However, it is actually now defence that is raising the most questions. The pass defence in particular has worsened over time. Key personnel lost on defence will need to be replaced effectively going forward.
All this provides a low starting point for Chip Kelly to take over.