One good way to emphasize the lack of "non-great Slam threats" is to compare the French Open over different eras. For instance:
2005-25 (21 seasons): 5 winners - Nadal 14, Djokovic 3, Alcaraz 2, Federer 1, Wawrinka 1
1989-04 (16 seasons): 11 winners - Kuerten 3, Courier 2, Bruguera 2, Chang 1, Gomez 1, Muster 1, Kafelnikov 1, Moya 1, Costa 1, Ferrero 1, Gaudio 1
Five less years, more than twice as many winners. Now part of that is the greater court homogeneity of the Big Four era, but not all of it. Some of it is also Rafa swallowing whole all clay specialists. But I think most of it is just natural fluctuation of the degree to which greats dominate the sport. From 1974-88, Borg, Wilander and Lendl won 12 of 15 French Opens, with Panatta, Vilas, and Noah sneaking in lone titles. Before Borg's reign, it was five different players in the first six seasons of the Open Era.
The point being, the tour has fluctuated in terms of "top-down hegemony" - not just on specific surfaces, but the tour as a whole. The first six seasons or so--aside from 1969, at least--were very balanced. You had seven different players winning multiple Slams in 1968-73. You had fading inner circle greats like Laver and Rosewall joined by lesser/near greats like Newcombe, Ashe, Nastase and Smith - and even Jan Kodes won three in that era. But then Borg and Connors rose to dominance in 1974 and the "superstar era" began. They were followed by McEnroe and Lendl. As Borg retired and Connors and then McEnroe faded, Wilander, Edberg, and Becker rose to replace them. Then as Lendl and Wilander faded, Agassi and Sampras replaced them (and Courier, for a few years).
But as Edberg and then Becker faded, no new greats emerged for quite a few years until Roger and then Rafa. From 1997 to 2002, only two 6+ Slam winners won Slams - Agassi and Sampras.
Meaning, from about 1974 to the early 1990s, the sport was just as dominated by superstars as it was during the Big Four era, but rather than three of extreme longevity, you had a shifting cast of 6-8 greats (those two extras being Sampras and Agassi coming in at the end). But then in the 90s to early 2000s, it was more of a "stars and scrubs" situation, especially after Courier dropped out of elite status in 1994.
But if we consider a spectrum from "top-down heavy" to "top-down light," in terms of how much the big titles are dominated by great players, we seem to be in a new era that is evocative of the Fedal Era (2005-10), with two guys winning almost everything. Novak and Andy started winning Masters in 2007, just as we saw the Big Four era begin to erode in 2017 from the bottom up as Murray dropped and Next Gen started winning Masters tournaments. But the Big Three held on, and it was only last year that the era truly ended...even if Novak is hanging on by his fingernails.
