Well the athleticism argument really doesn't apply to Jordan: he was as athletic as anyone who has ever played the game. But still, in a more general way I'd disregard the athletic argument for a few reasons.
1) While Kobe and LeBron are obviously super-athletic, guys like Steve Nash, Tim Duncan, Dirk Nowitzki, and Paul Pierce aren't, and they're still dominating this league. So it obviously proves you don't need to be an athletic freak to be a great player in today's league.
2) It's not like there weren't great athletes in years past going back to guys like Dr. J., David Thompson, and Elgin Baylor. And while all were great players, it's not like they were scoring 100 ppg.
3) If any old player played today, we'd have to factor in the improvements they'd get in conditioning and training in today's league. Bill Russell was listed at 6-9 220, which would be considered undersized for the power forward spot in today's league, let alone the center spot he played in the '60s. However, you put him in today's league, you can pretty much count on him having another 30+ pounds of muscle that he'd get from an improved diet and weight lifting program. Similarly, LeBron James wouldn't be built like a linebacker if he played 50 years ago.
Red Auerbach once said something to the effect that today's NBA doesn't have better players than it did when he coached Russell and Cousy, today's NBA just has more of them. I tend to agree with that given the fact that more kids grow up learning to play basketball now than they did in the '50s in more parts of the world. Thus, the talent pool is bigger than it was then to choose from. But I also agree with Red that the truly elite players from his era could have dominated in our era as well.
To take his theory one step further, I think you can also make an interesting evaluation of the league by comparing the amount of talent in the world to how many teams there were in the league at a given standpoint. In my opinion, the league pretty much kept up with the expanding the league to accommodate the amount of talent available to it through the '80s. However, in the '90s I think we saw the low point of basketball with the expansion teams coming and diluting the league. However, I think over the past 5-10 years or so, I think we see the league slowly starting to catch up as more great players from around the world come into the league and (for the most part) the league is limiting expansion.