Djokovic seems like a nutcase with many of the comments he makes about new-age stuff (water purification through emotions, placing bread on his stomach showed his gluten sensitivity, no-vax, energy pyramids, peace and love through hugging etc.) and it makes me feel that he can easily get enthralled by another conman like Pepe in the future also and waste future years of his career like in 2016-2017.
Djokovic's 2016-2017 was in my eyes by far the biggest slump that any of the three had, so much so that I was absolutely certain that he wasn't coming back from that. I doubted his return much more than I doubted those of Federer and Nadal at any point, including current Federer's injury at the tender age of 39. Were such slump to happen again, I would even say that his chances of catching Nadal are flat out zero. But I don't believe that it will happen again, not nearly to that extent anyway. And as skeptical as I am personally of many of the Djokovic's unconventional methods, some of them seem to work for him, for the most part.
However, being considered the GOAT largely depends on career accomplishments and in my opinion, Djokovic has had too many down periods for Slams (2004-2010, 2012-2014, 2H2016-1H2018) compared to Nadal who has been more consistent.
I'm not sure about 2004-2010. Nadal turned pro in 2001 and won his first FO in 2005 at 19. Djokovic on the other hand turned pro in 2003 and turned 19 in 2006. So I wouldn't consider Djokovic's 2004-2005 relevant in this context, unless we want to call Nadal's 2002-2004, or at least his 2003-2004, a down period as well. But I think that it would be too harsh. As for 2005-2010, sure, Djokovic didn't make a complete breakthrough. Federer and Nadal were just that much better. Federer was obviously in his very prime. As for Nadal, additionally to being a year older (two in pro-tennis years) than Djokovic, he seemed to have matured at a younger age. We might however see a reversal of that at the respective ends of their careers. All things being equal, wouldn't it be logical to expect Nadal to also enter his decline 1-2 years before Djokovic.
As for 2012-2014, I don't consider it to be a down period for Djokovic at all. I mean, he won 3 GS during that time and played 5 more finals (I agree that it's not optimal to lose so many finals, but is it better to reach a final and lose it or not to reach it in the first place). Murray had one of the two peaks of his career during that period. And after all, Nadal won only one more GS than Djokovic during that period, 4 that is, while reaching just two more finals.
So, no reason to assume that Djokovic can easily make up the 3-Slam advantage that Nadal still has.
For me, Djokovic being a bit younger and less spent accounts for the advantage that Nadal currently has. I'm not advocating that Djokovic will overtake Nadal, just that they are actually very comparable at these junctures in their careers.