The facts are clear enough. The period of 2013 - 2018 has been very weak regarding the field relative to previous generations of players, probably back to the first few years of the open era. Nobody under the age of 31 has won a major. Players born from 1989-1995 should have started peaking between 2013 and 2019, reaching their 24th birthday and been winning majors. Yet they haven't won even one major. They haven't pushed out the much older generations of players, and therefore allowed them to accumulate more majors.
The older generations' great players (Djokovic, Nadal, Federer) have taken advantage of these players incapable of winning even a single major.
The youngest of these players, Djokovic, has naturally benefited the most without a single great player younger than himself. Nadal close behind, and even much older Federer has taken advantage by winning 3 majors in 2017-2018 when 1993-1994 players should have been hitting their peak at 24. Shockingly, no players even good enough to win a single major came along.
Now imagine for a moment how many majors Federer would have won if Nadal and Djokovic had not been around. Probably 2006-2007 would have been Grand Slam years and one shudders to think how many majors he might have accumulated until losing motivation.
Federer has had much younger great players dogging him since 2005, his 24th year, starting with early blooming Nadal, then Djokovic by 2008 all the way to the present time. That he is even competitive with his significantly younger superb opponents at his age speaks to his overall greatness. By the time Nadal turned 24 in 2010, and Novak turned 24 in 2011, they were peaking as Federer turned 29-30. Federer won Wimbledon in 2012 at close to 31, then had a dry spell for years, as peak/prime Novak and Rafa took turns dominating with contributions from Murray('87) and Wawrinka('85). The 1988 generation fizzled with the injury troubles of del Potro, and the flash of Cilic. Rafa turned 29-30 in 2015-2016, and Novak in 2016-2017 and were ripe for being passed by younger peaking generations of the early 90's, but there were simply none to be found, so these older players eventually returned to form and continued winning and alternated among each other winning an additional 3-5 majors.
There are signs that this dearth of major winners is finally about to change. The 1996-1999 players along with late blooming Thiem('93), are winning big titles like the Tour Finals and Masters1000 here and there and are beginning to show consistency needed to rise high in the rankings, just below the Big 3. Still, as of now, they haven't won a major. However, this age group should start winning majors, as they move from their very early 20's to their peak years. It should happen very soon, 2020, 2021 at the latest, and a new #1 should be crowned when that happens. If it doesn't happen, take away their mobile phones.
But seriously, once that does happen, don't be surprised if the big 3 fade from the scene rapidly. They have had their time, even much more.