Well, that's not really apples to apples though. As Bill Simmons said, Bird came at the end of an era for the SF position. Athleticism followed and after Rodman, athletic lanky forwards gave him considerable troubles. Was George Mikan athletic?
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Rebounding is one of the biggest indicators to athleticm. There are exceptions to the rule (Love, Reggie Evans) , but the situation is that Durant has athletic markers all over the place. Shaq was athletic, but not because he ran a 4.4 40. Shaq was athletic because he could do things no 7' guy should be able to do. Same with Durant.
I think there are several axis along which one can be considered 'athletic' and I don't think folks will reach consensus on which is most important.
You seem to think that it correlates with what I'd call 'gymnastics' athleticism, which is a reasonable take. Its hard to argue that gymnasts aren't athletic so it makes for an easy to understand standard. Leaping, running, strength, agility. Pretty obvious attributes.
Here is some fodder for thought:
Did you know that Jeff Green measured out with a 33.5" no-step vertical at his draft combine? That's more than SEVEN INCHES HIGHER than Kevin Durant measured (26.0)! Green also had a significantly higher 'max vertical': 38.0" to 33.5".
Green's 3/4 sprint was also significantly faster: 3.34s compared to 3.45s. Green ended up ranked 18th in overall combine marks at that draft while Durant ended up 78th.
This was clearly way back at their combine - so how relevant those numbers are at today is of course arguable.
But at least at that time, its hard to make an argument that Durant was an elite athlete and Green was not - along that particular set of axis.
I think, though, there are really some broader attributes to athleticism than just those.
Speed. Strength. Quickness (different than speed). Body control. Hand-eye coordination (again, different from body control). Stamina. Other things not in my head at the moment.
Someone could be 'elite' at some of these and not others.
Are Olympic archers elite athletes? Some looked a little soft & pudgy. But they can do something no other athletes can do because of their skill and precision.
I think George Mikan WAS an elite athlete - he had superior strength, endurance AND body control, to go along with his raw size.
I think Larry Bird WAS an elite athlete - for similar reasons - body control, hand-eye coordination. He may have lacked speed, but had amazing quickness.