Yeah, you left out the part that most of the players in the still desegregating, not very financially lucrative league in which many players had to have off-season jobs to pay the rent, and in which the pool of players worldwide was several orders of magnitude smaller than it is today, resulted in an NBA back then where most players probably couldn't even make a D-league roster today. But it's tougher because something something Will Purdue (who would probably be an all-star big man back then) - god forbid you gotta go against the badass bigmen of the early 60's like Dolph Schayes (OMG - how would Shaq deal wih all 6-8 195 lbs of him!!). Shaq was able to physically dominate much better, bigger, faster, stronger athletes in the 2000s - when you match up against the skinner smaller slower not-very-aggressive defenders of the 60's, it would be a war crime akin to having Usain Bolt run against 1960 Olympic Champion Armin Hary (Ger) and his 10.3 second 100 or having the 66 Packers try to beat the 13 Seahawks. It's not happening, end of story. In reality - Human athletic performance increases over time - combine this with the fact that athletes from pretty much every single sport in which particpation increases over time (if only as a function of population growth...) will be drawing from a far deeper talent pool than their predecessors. It's just math.
