Djokovic's loss of motivation is quite clearly a big loss to the game.
Also, not sure what Federer you were watching but he most certainly wasn't crap at 19/20. By the time he was 19 he had already shown more in grand slams than the likes of Kyrgios, Zverev, Dimitrov. At 20 he had already won a grand slam ffs.
I still hold out hope for Zverev but the other two are complete write offs. Kyrgios is a biggest disappointment given he's probably the biggest talent of the 3. Zverev has shown nothing in grand slams whatsoever so far to inspire any confidence though.