I will Concede On Zverev if he doesn't make any inroads in the Slams over the next 24 months, I personally think that will change though given his results in masters and world tour finals. He's still 21 and I think it's just a matter of time before some of that starts to translate into the Slams.
I've always felt that narrative was a bit trumped up personally. His rise was steady, and then when he flicked the switch in 2004 he arguably exceeded expectations (had rarely seen a player dominate a season like that, and he was just getting started).
2000 he was a next big thing kind of talent, gave Australia a massive fright in the Davis Cup 1st round. Wins the 2001 Hopman Cup. Wins his first title against Boutter (still fondly remember both). Makes a Wimbledon QF beating Sampras. Halves his ranking. 2002 he halves his ranking again, the main knocks is that early exit at Wimbledon and developing a losing streak against Nalbandian. In 2002, 3 years after he entered the top 100, he makes the SF of Masters Cup. 2003 he wins a slam, Masters Cup, and is on the verge.
People today might look at the meteoric rise of Nadal (effectively bolted to a slam within 2 years) and Djokovic (think of him pre-FO 2006 to Indian Wells/Miami 2007), or even at the time Safin, Hewitt and Roddick, and wonder what took Federer so long, but at the time his rise felt very steady. Was around for a few years before dominating. You'd take that any day of the week. If he got any criticism, it was probably reacting to slip ups at specific events than any broader assessment.
His loss at Kooyong 2004 to 33yo Agassi was the last time commentators really had a chance to naysay his deficiencies (was beating himself up during that match).