I'm just pointing out that the vast majority of teenage Slam winners were in the 80s (or 82-90, to be exactly). 7 of 11, to be exact - 2 before, 2 after that period of time.
You have Borg's two in 1974-75 on one end, then seven by Wilander (2), Becker (2), Edberg, Chang and Sampras in 1982-90; then a gap until Rafa in 2005-06 (depending if you count age at start or end of tournament), and a similar gap until Alcaraz in 2022.
The point being, outliers will occasionally happen, but there was a high concentration in 1982-90...for whatever reason.
And of course only four players have won a Slam at 17 years old: Borg, Wilander, Becker, and Chang -- and none since Chang in 1989, 36 years ago. Hard to imagine the 17-year old that would break that streak in today's era. It is possible, I suppose, but...
Anyhow, when Rafa won Roland Garros, I think it was simply a matter of exceptional talent that bloomed early than a lack of "cowing." It was not long after several 20-year olds won Slams: Kuerten in 97, Safin in 2000, Hewitt in 2001, and not long until Novak won his first at 20. I'm not sure there's something magical about turning 20 that avoids "cowing," but more that Rafa was just that great at an early age.
Of course Fonseca hasn't won a Slam yet. He's got six more to play as a teenager, and given his meteoric rise, it is certainly possible. I don't think this as much signals a breaking of the dam, as a third exceptional ("all time great") talent to emerge in the last few years. Talent comes in waves, and I just don't think any of the players born 1988-2000 were as talented as Sinner, Alcaraz, and seemingly Fonseca. I think there are some underachievers in that range, but I just don't see any of them being on the level of the Classic Big Three or the New Big Three. Nishikori, Raonic, and Dimitrov might have won a Slam or two and/or some big titles in a different era, but none would ever have been ATGs. Similarly with Thiem, Medvedev, Zverev, etc. All very good players who could have won more if not for the shadow of the Big Three, but none scream "ATG talent" to me - not like Alcaraz, Sinner, and Fonseca do.
(Again, assuming Fonseca becomes as good as we hope).