Yeah, unlike Nadal, djokovic hasn't decline physically. IF there's someone who can beat 18 majors, it's djokovic IF he stays injury free. I don't have faith in the young guns to be honest. There's nothing remarkable in any of them. I don't see greatness in any of them.
When Fed was 20, you saw footwork, forehand, speed around the court, great serve, amazing touch and manufatured shots you've never seen before.
Nadal at 20, held 2 majors and several masters and already world number two. A true tennis prodigy.
Djokovic at 20 was a major winner with several masters.
Sir Murray not as successful as the other three but at 20, you saw a dominating backhand and court coverage that could only be rivalled by Nadal and Djokovic. He started out winning masters and somehow you knew he was greater than just a collector of masters titles.
What those 4 have in common is the fact they possess several traits that you could say "it belongs to one of the greatest ever".
Federer: forehand, footwork, passing shots, all-court tennis
Nadal: forehand, passing shots, footwork, speed, court coverage, turning defense to offence, mental stregth
Djokovic: backhand, court coverage, return, speed, endurance and mental strength
Murray: backhand, return, speed, court coverage
Now tell me 2 things in (Raonic, Nishikori, Dimitrov, Thiem, Zverev or Goffin's) game that you could say "that's one of the greatest ever".