Djokovic is going to have the toughest year of his life next year.
He's basically starting off 2016 in a pure defensive mode: he's got to defend all of his points from 2015. This means that at best, he won't win hardly any more points next year. At worst, he will only lose points.
Since he made the finals on almost every tournament he played, any time he doesn't make a final, he loses a ton of points.
Any time he decides to skip a tournament that he participated in 2015, he loses a ton of points.
Any tournament he doesn't win next year, he loses a ton of points.
It's going to be the most stressful year of his life, and everyone's going to be gunning for him. With so much to lose, expect gamesmanship to go up. People will push him to his limits, knowing he's got so much on the line.
There is absolutely zero chance he will win all of the same tournaments next year as he did this year. 100% guarantee that he's going to finish lower next year. May not drop from #1, but the gap will be significantly narrowed.