None of them are younger ATG's. There are no all-time greats younger than Djokovic right now, which speaks half to his and Nadal's longevity, but also undeniably to the weakness of his younger competition.
Pretty much this.
The last 3 out of 4 slams at least had a NextGen in the final and in 2 they at least made it close. Thiem on Clay has been the only consistent threat from Gen useless or NextGen. Here’s a breakdown of possible slam winners over the last decade if you just took the big 3 out of the draw at the last stage where they beat someone other than each other.
AO20 - Thiem, Sandgren, Raonic
US19 - Medvedev, Berrettini, Cilic
W19, RBA, Querrey, Nishikori
RG19 - Thiem
AO19, Tsitsipas or Pouille, but they won 10 games between them in the semis
US18, Del Potro, Thiem, Nishikori
W18, Anderson, Del Potro, Nishikori
FO18 - Thiem or Delpo
AO18 - Cilic or Berdych I guess
US17 - Anderson or Delpo
W17 - Cilic or Berdych
FO17 - Wawrinka or Thiem
AO17 - Wawrinka or Dimitrov
US16 - Wawrinka
W16 - Murray
RG16 - Murray, Thiem, Berdych
AO16 - Murray, Berdych, Nishikori
US15 - Wawrinka or Cilic
W15 - Murray, Gasquet, Cilic
RG15 - Wawrinka
AO15 - Murray or Wawrinka
US14 - Cilic
W14 - Dimitrov, Raonic, Wawrinka
RG14 - Murray, Ferrer
AO14 - Wawrinka
US13 - Wawrinka, Gasquet
W13 - Murray
RG13 - Ferrer, Wawrinka
AO13 - Murray, Tsonga, Ferrer or Berdych, Wawrinka
US12 - Murray
W12 - Murray
RG12 - Tsonga, Del Potro, Ferrer
AO12 - Murray, Berdych, Ferrer
US11 - Murray, Tsonga, Tipsarević
W11 - Murray, Tsonga
RG11 - Murray, Soderling, Monfils
AO11 - Murray, Berdych, Wawrinka
US10 - Youzhny, Monfils, Soderling
W10 - Berdych, Murray
RG10 - Soderling, Melzer, Almagro
AO10 - Murray, Tsonga, Davydenko
Del Potro, Cilic are 1 year younger than Djokovic. Those two plus Nishikori first showed up in an important way 2008.
Murray, Wawrinka, Anderson, Berdych, Tsonga, Ferrer are older than Djokovic.
Obviously Djokodal have been amazing, but Thiem is the only younger gen guy that shows up as a multi slam threat, and even he seems to lose his big matches to less accomplished players (Tsitsipas, Zverev recently).
To put it another way,
Thiem is the only guy to emerge in the entire decade of the 2010s to threaten for slams.
On the flip side, here breakthrough dates in the 00s, eventual slam winners only:
00 - Safin
01 - Hewitt
03 - Roddick/Federer/JCF
05 - Nadal
07 - Djokovic
08 - Murray
09 - Delpo
The narrative that the big 3 are just superhumans holding back a reservoir of younger gen talent is obviously totally wrong.
The other thing to note here is that Murray’s peak ended after W13. Could have won up to 11 slams between AO10 and W13 without the big 3. In the rest of the decade he had 6 chances (won 1).