And yes I get the counter argument that one day it’s likely a Zverev/Tsitsipas/Medvedev will reach 100 weeks at #1 eventually. Competition is of course a big factor and Nadal has put in some great years only to be 2nd to another ATG in one of his peak seasons, for example.
However being #1 is not subjective. It requires health, consistency, and a level of discipline, professionalism, and innate competitiveness to bring it every single match that many of these player lack. I’m not talking about a <50 week #1 like Rios or Muzzah in a vacuum situation. I’m talking about people who have defended their #1 and fought to get it back once they lost it (Djokovic at ATP Cup 2020 vs Nadal is a great recent example of a match for de facto #1). Those matches used to be regarded as some of the biggest in the sport, keeping or losing #1. Now they are “not relevant to the GOAT debate”