I don't see how it's that much harder for the young players to break through today than it was for Djokovic, Murray or Del Potro.
.....Sorry Meles I just don't see anything special in these young players and believe me I've followed this sport very closely since 2006/2007. Zverev is the only player I feel would matter back in the day. He'll figure it out in the Slams. I had high hopes for Kyrgios but he's just wasting his talent. As for the rest - maybe they will mature later on but it's not like they are 17. They are 19/20+ years old and they should be having a lot better results at this stage if you think they are 4/5+ Slam material. I honestly think they aren't even better than tier 2 players like Berdych, Nishikori or Ferrer. Someone like prime Tsonga would love to play today and bag a Slam or two. Instead it's the same old for 15 years.
I'm not even going to compare Bendy, Nishi, or Ferrer because they just don't have remotely similar accomplishments at the same age.
Let's do take a look at the Golden component of the diamond age. I'm willing to write off Murray at this point because his comeback from hip surgery has just been too slow. Wawrinka may yet have some fight left in him and I think Magnus Norman back with him means they think something is possible. Looking at his recent results he's clearly at least at top 20 level.
Federe still will have opportunities at Wimbledon and the upcoming Auz Open for sure, but I think we can write off the US Open at this point. He's not been NextGen resistant enough and they are good on outdoor hard courts. I thought he'd play until age 40, but now I expect to hang around in top ten through Tokyo. With Djokovic back on the prowl it would be a miracle if he got two more slams.
Nadal will be player to beat for 2019 RG, but I think the cracks will really be apparent that his days are numbered on clay. Thiem, Zverev, Tsitsipas are big threats and will be on close footing with Djoko on clay in 2019. Coric may finally emerge as a real clay threat in 2019. Khachanov has done well enough to have hopes of taking another step. Shapo should continue to develop on clay nicely. I'd say two more RGs tops for Nadal and maybe somehow a total of three slams.
Djoko will have a lot more staying power than Nadal. He'll be the favorite at Wimbledon and I don't see any of the young players ready to cause trouble in 2019. Somebody has to win 2019 Auz and if its not Fed, Djoko looks good. Tsitsipas is a bit of an unknown at Auz so hard to see him contending. Shapo might suprise because really his early 2018 showed great potential:
3 / 2018 Australian Open
R64 Hard Jo Wilfried Tsonga Denis Shapovalov 3-6 6-3 1-6 7-6(4) 7-5 H2H 1.69 - 2.10 (Tsonga had won 5 tournaments in 2017 so no slouch at this point.)
R128 Hard Denis Shapovalov Stefanos Tsitsipas 6-1 6-3 7-6(5) H2H
2 / 2018 Auckland
R16 Hard Juan Martin Del Potro Denis Shapovalov 6-2 6-4 H2H 1.33 - 3.20 (won Auckland in top form)
R32 Hard Denis Shapovalov Rogerio Dutra Silva 6-3 6-2 H2H 1.30 - 3.40
52 / 2017 Brisbane
R32 Hard Kyle Edmund Denis Shapovalov 6-7(5) 7-6(4) 6-4 H2H 1.91 - 1.91 (SF at Auz)
Lets say Djoko and Fed split Auz Open 2019. We won't know for sure until Djokovic produces results on the new, faster surface. With US Open set to be a touch slower with the changes this year. Djoko the favorite in 2019, but will have to see how US Open series hard courts goes in 2019 which has been a great surface for NextGen.
Slam outlook:
2019 Auz Fed/Djoko/Nadal
2019 RG Nadal or other if Nadal unhealthy
2019 Wimby Djoko
2019 US Open Djoko
2020 Auz ????
2020 RG Thiem or NextGen or Nadal
2020 Wimby Djoko
2020 US Open ????
2021 All NextGen (or Thiem) except possibly Wimby
I definitely would favor Djoko for those questions marks and something in 2021, but we won't really know until we're through 2019 Auz.
Do you see more beyond this? Do you see LostGen (and I include Cilic and Delpo) or Isner/Anderson somehow winning a slam? Maybe one LostGen slam is the most I see. Somebody has to win slams in the 2020s and its pretty clear in my mind that it will be the NextGen players I identify and perhaps a few for LostGen and Thiem.