It is unrealistic to expect each generation to produce players with the Big 3 calibre or talent. They are more than mere tennis players: they are tennis geniuses. We are witnessing the 3 best tennis players ever battling in the same epoch. Players with the Big 3 talent emerge once in a millenial, in the same sense that composers such as Beethoven, Mozart or Bach emerge once in a millenial, not every 5 or 10 years. Another example: Socrates, Plato and Aristotle were alive over 2 millenial ago, yet the world never witnessed any philosopher comparably as great till Immanuel Kant publsihed the Kritik der Reinen Vernunft (1781). 2 millenial were necessary to produce another philosophical genius of the same order.
ATGs from other eras could have failed to become ATGs in the Big 3 era (roughly 2003-2021). Maybe Becker or Wilander would have had a Murray-like or Wawrinka-like career playing against the Big 3. Maybe they would be even Slamless. We will never know. The point is that it is unfair to compare other eras with the Big 3 era. Other eras produced more ATGs precisely because no one was good enough to dominate the field like the Big 3, nor did anyone from previous eras display the Big 3 talent. The only exception could be Borg, but we will never know because he retired early and it will remain as a "what if?" situation.