2019 Djokovic is still an "old" Djokovic though, and does not compare favourably to any actual prime years from Djokovic. And when Djokovic starts losing more consistently I expect a lot of Djokovic fans will start saying many of the same things that the Federer fans here are saying. The things that are being disagreed upon now will come back around and probably be agreed upon in future when Djokovic fans are wondering if some 25 year old kid beating a 39 year old Djoker would beat him in his prime or even when he was in his mid thirties.
Because right now, this isn't prime Djoker and he's not improving by the day just because he's still winning slams. What Djoker is doing now is what Federer would've been doing in 2015 if not for Djokovic himself. Maybe I'm stubborn, but you'll have an exceptionally hard time convincing me otherwise.
As for Thiem, the problem here is that while Thiem's career is already better than most of those guys, why is that? He's very good on clay so he's made a few RG finals, but on HC and grass he isn't really that special. The problem for tennis post Big 3 is that it looks like the actual level required to win a slam, and beat guys like the Big 3 is going to go down. If Thiem beats Zverev to win a RG or two in future and the Big 3 are retired or legitimately washed up so much that they aren't really a slam winning threat, does that make Thiem better than Tomas Berdych or Cilic, or Roddick for example?
We can bible thump stats all day long and say well "this guy has 3 slams and this guy has 1, 3>1 QED" but it all seems a bit too simple and hollow to me. But that's just my 2 cents. I don't want to force my opinion on anyone, and I honestly don't have an ulterior motive in saying this last bit. The "GOAT" race or whatever the F it's being called now bores me to tears, so I'm specifically talking more about the future of tennis here. It doesn't sit that well with me that Thiem can be relatively unimpressive for much of his career so far, especially on grass and HC, and will probably end up with more slam wins than the aforementioned guys simply because the timing works out better for him.