lindsey, we have to take into account age and the context of play. Novak reached his peak form in 2011 at age 23-24, then dropped a bit for a few years in a high plateau in which he was "first among equals," aside from 2013, from 2012-14 (actually, it was really only the first half of 2013 that Rafa was ahead of him). Then he surged in 2015, at age 27-28. This was partially due to him consistently finding a high level of play, but also because 2015 was a pretty weak year for the tour: Andy and Rafa struggled, and Roger played well but clearly wasn't what he was, and the younger generation had never really risen to challenge.
Novak is turning 30 years old in less than two months. He is young enough that I think he can return to an elite level, but I highly doubt to an utterly dominant form like he was in 2011 or 2015. I think the best case scenario is something similar to 2012-14. In fact, I would be surprised if he didn't win at least a couple more Slams and continue to be among the very best players in the sport for two or three more years. But there are signs for concern.
As I see it, there are three factors that are working against him. One, his mentality - I'm not sure how into it he is. Maybe it is just bravado, but he also could have lost the drive to win.
Secondly, his movement. I'm wondering if he has lost a hair of movement, which is impacting him. I haven't watched him enough to really say, but he might be a tad slower and less able to retrieve everything (maybe someone who has watched him a lot this year can comment).
Finally, the young players. Kyrgios and Zverev will only get better. I think that, along with Raonic, Nishikori, Dimitrov, and Thiem, they form a strong new "second tier" of players - and in a year or two, not only will Kyrgios and Zverev become first tier, or elite players, but more young players will rise to expand the second tier into a rather deep pool of players.
I suppose we can add a fourth element, which is his peers, the Big Four. If Roger maintains his current level he's going to be very hard to beat for the rest of the year. Rafa is looking to surge during clay season, and Andy should return to form.
Combine all of these factors and I think the sentiment that his reign--that is, rulership as "king" of tennis--is over, looks even truer now than when I wrote the original post a couple months ago.