There are clearly signs that Djokovic is in decline, and is vulnerable on faster surfaces. I don't think he'll ever have a season like 2011 or 2015 again. The question is how it will affect his ranking. Until the French next year, he'll be more susceptible to early losses than this year, so until then he's more likely to drop points then he is to gain then. That's one variable in the equation. The other is about who is gonna take it away from them. IIRC, if you made look at the race since MC, Murray is actually ahead of Djokovic. Now Djokovic is a bit lucky that Murray seems tired as well, so I'm not that sure he'll capitalize on Djokovic having form issues, but I really think that if Murray will bounce back fresh next year and be ready to make a push for #1, especially if Djokovic were to carry his form issues through the winter.
In the race for World #1, Djokovic has been incredibly lucky, with this year being an Olympic year, Murray being dead set on playing Davis Cup, and his absolute cake walk draw to the USO finals. Djokovic got 1000 points of advantage for nothing in Toronto, and the Olympics clearly tired Murray out.
If Murray can't take advantage, I think it will take a fair while. Wawrinka is unlikely, Fed and Nadal out of question, Raonic and Nishikori aren't happening as well. Then we're looking at players who are barely top 10 yet. I really think that as soon as Djokovic starts losing the close calls he's had this year with the likes of Nishikori and loses the odd 2nd round match, he's there for the taking. Question will be when somebody is there to take it. So then you're down to the question whether a player stands up and is able to challenge the slams on all 4 surfaces. I don't see Zverev making a HC run before 2018, I don't see Thiem making a fast HC run before then either, and both have a long way to go anyway. I've been relatively impressed with Kyrgios' constistency this year, he tanked less then I expected him to do, but he also didn't make any deep runs and had bad luck with draws, injuries or lost a crucial match any time he had a shot at going very deep in a slam or 1000. Kyrgios has the game to challenge on HC and grass, and is actually reasonable on clay, but he's losing too many matches to the likes of Berdman and Nishikori in crucial moments, and keeps drawing Murray in slams who he doesn't believe he can beat.
In short, Djokovic is incredibly lucky. He's been very consistent, the players who could challenge his position short term are either his pidgeons, declining harder, inconsistent, and his main challenger has had a matchup issue. The youngsters are probably long enough off for Djokovic to get the record.
Now there's one player I haven't discussed, and I'm slowly starting to believe in him for #1 next year.
Pouille in 5.