I think people underestimate the effect of television too ... imagine being an athlete in the 50s/60s any basically never having seen some of your opponents play or only seeing it maybe once a month at best, lol. NBA was basically not televised except for the Finals until the 70s.
The talent pool for sports is so much bigger as TV become a staple and all games were televised. You could study/watch the greats, get inspiried, want to be like that, etc. etc. etc. Today an athlete watches constant footage of other competitors, breaking down every little nuance of their game and adjusting their training towards that, you have guy's like Kobe who obviously watched every little thing Jordan did for example and copied that. In the 60s half the players didn't even train in the summer (that's what training camp literally was for), they had to have part time jobs to pay their bills.
How can you compare that to a modern player who trains 2-6 hours almost every day in the summer with regimented diet and hours also spent in the weight room?
Wilt, Russell, and Jerry West IMO could still play well in today's era, though not nearly as dominant, but a lot of other guys from the 50s/60s would be completely exposed as mediocre players if they had to play against modern talent.
By the mid 60s they were airing TV pretty regularly. The 50s and 60s are not the same era, btw. They are completely different. Startin in 58 or so, the game changed in a really, really fast way. The game played in 65 was nothing at all like 55.
Also talent pool was smaller but the league was a fraction of the size. So talent pool relative to available positions was not as different as you think. There were 8 NBA teams in 1960 and 14 in 1969. That's a fraction of 1996's 29. So fewer players but also far fewer teams. That's how you could wind up with a team like the Celtics, where a player like Havlicek is a sixth man. Also, going back to the TV thing, this also meant that was less of an issue. Because you still played 79 games in 1960 and 82 games in 1969. So you didn't need TV as much, you played these people crap ton of times.
And on the pay thing, the best players did not need that at all. Again, you are using 50s info and applying it to the 60s. No they weren't filthy rich, but they made a living. Players did retire earlier, but that was largely due to the physical nature of the league. No flagrant fouls meant the best players took a really bad beating.
As for how players would do today, you have to look back and ask if their dominance was due to the state of the league or in spite of it. West was not a 60s player. He was undersized and playing in a league where it was all about being strong and playing physical in the paint. He was a great range shooter in a league where a 3 point shot was just a low percentage 2. He was a great on ball defender in a league where the pace of the game made it so he couldn't make use of that skill. West would actually be a far better fit for today's league than the 1960s. And Russell and Wilt would be great because the foundation of their greatness was athleticism and basketball IQ, which is forever. WIlt Chamberlain played in a league where he was pummelled constantly, had to play at a very fast pace, and was the primary scoring option for most of his career. And he averaged over 45 minutes played a game. In 61-62, he averaged 48.5 minutes per game. That is, more minutes per game than in a game. And though not quite as in human, Russell was similarly built on athleticism and endurance. For those kind of players, the current league with its gentler play and slower pace would be right up their alley. Especially Wilt, who was also a very clean player, never fouling out once in his career. I mean, I would just say in general that athleticism is the least of the problems your 60s greats would face.
But yes, some players would struggle. As is the case in most eras, your mid tier players would really have a hard time because a lot of them took advantage of calling tendancies and such. But I would say that Wilt, Russell, Oscar, West, and Baylor would all fit in very well in our era. And Havlicek as well. I mean shoot, Havlicek went on after Russell retired to lead a new Celtics squad to championships in 74 and 76, so his success was definitely not due to the 60s era. And btw, that speaks to the quality of the 60s Celtics since he was coming off the bench for a good part of the 60s. The all time leader in total points scored for the Celtics was their sixth man.
Point is, in any era there are a lot of players who wouldn't transplant well to another because their success is predicated on the particulars of that league. That's why analysis of "greatest of all time" and trying to figure out how great a player is outside of that context is challenging and interesting.