They will probably still get 2-3 I would say and one of the up and comers with a strong mentality can steal one or maybe 2 away. I think the hard courts ones are up for grabs but I don’t see RG nor Wimbledon being won by anyone but a big 3 member.
(1) AO: Djokovic or Rafa
(2) FO: Rafa
(3) WIM: Djokovic, Federer, or Rafa
(4) USO: Rafa, Djokovic, maybe, NextGen, or old horses, Wawrinka, Cilic, del Potro, Murray.
The man NextGen will have a hard time beating will be Rafa. If NextGen takes out Djokovic and Federer, Rafa is the guy who will cash in and win the title. In 2019, his two slam losses came in the hands of other two Big 3; Djokovic and Federer. Since 2017, except Wimbledon'17 loss to Muller, all the remaining slam (6) losses came in the hands of prior grand slam winners; Federer, Cilic, Djokovic, del Potro, which period also ended up in giving him 5 grand slam titles. History shows that experienced campaigners (which also include Muller) have a chance against Rafa, not the newcomers.
people think that this season is different. actually, this is going on since 2016 when NextGen Thiem ended the year in top 10. and the predictions at the off season came in. 2017 comes with even older gen (Fedal) winning the Slams. and boom - Dimitrov wins WTF and everyone climbs to Gen U bandwagon. "Dimitrov will win AO 18" they said. 36 year old granpa wins instead. fast forward WTF 2018: "this is my f*cing time!"... again Big 2 dominates. for me, the real change will be in 2023 when Djokovic reaches 36 and unable to win Slam. until that, off-season beckons!