There is no one prototype way to be successful in the NBA. You can't just get by on athleticism, you can't just get by on size/length, and you can't just get by on skills. Being better in one department can make up for lacking in another department. This concept should be very simple. Just like there are many categories in the skills department (jump shot, dribbling, ball handling, shooting, footwork, post moves, touch, vision, decision making etc.), there are categories in size and athleticism. Saying a player does not have explosive leaping ability or quickness, is not the same as saying a player is a poor athlete. Just like saying someone is a bad shooter, is not the same as saying they have poor skills.
The difference however between skills and size/athleticism is that you can't learn size and athleticism, but you can learn skills, so it affects a player's ceiling whether you like the idea of ceiling or not. If you have a player that is super skilled but severely lacks size and athleticism, there isn't a whole lot they can do to improve their game. On the other hand, if you have a player that has good size and is very athletic but severely lacks skills then there is a lot they can do to improve, they may not improve but there is a lot they can do to improve. Now these are obviously extreme examples and it doesn't even address who is better, but it points out the idea of ceiling and how different players don't always have the same room for improvement based on how their games are made up. Brandon Roy started out a very good player and he's gotten a little better every year, but he's still not likely to become that much better than he already is, which is an all-star level guard but not really a player that can carry a team.
Now as far as projecting improvement goes, you can't just look at things like players A and B are both bad shooters and they're the same age so they have the same potential to improve their shots. If a player A has good mechanics but still doesn't have great accuracy, you can project that he has a greater probability to improve his shot than you can with player B that has accuracy issues with poor mechanics. The likelihood of a player improving their mechanics depends on what the specific problems in their mechanics are and looking at history with players with similar problems and how they fared at improving them. It's a difficult process to change mechanics that are really messed up and sometimes it's not worth how much it would set a player back. The bottom line of a jump shot despite how unorthodox it is though is how high can get your shot off, how quick can you get it off (off the dribble and set), and how well you can sink it. If you can get it off higher than it can make up for whether it's a slow release, but it still limits the the contexts in which you could have gotten it off if you had both traits.
With rebounding. You can still be a good rebounder without a great reach if you have good athleticism and IQ, but you won't be able to do it in as many contexts than if you had all three because with reach all you have to do is lift your arms, but with athleticism you have to spend more energy and time leaping and moving.
It goes for finishing as well by making up for a lack of explosiveness by using reach, touch around the basket, and body control, but it still doesn't give them an ability to get high up on a big tall defender in traffic and go "over" him, and that doesn't even necessarily mean dunking. So their finishing ability ceiling is lowered because they can't do that extra thing no matter how hard they practice.
My point is that even though you can make up deficiencies in one department with ability in another, that doesn't mean that you can do things the same way. In basketball, there are so many different contexts for every little ability, and they're all apart of analyzing and projecting a player. It's not just about how many points a player scores, or rebounds they grab, or assists they dish out, it matters how they do it as well because there are advantages and disadvantages to doing certain things certain ways. You can't just look at how someone tests out in max verticals and say that's the quality of their athleticism, because there's more to it than that. That's them taking a step against no one and the quickness in which they launch makes no difference in that test, it does in the game. And personally, I think it's more valuable to be a quick leaper than it is to be a high leaper that takes long to gear up for the leap because that limits the number of contexts you can actually use it in a game. It's about beating your defender to a spot and how you beat them there, whether that spot is on the floor or in the air. Skills/fundamentals wise, Nash has the best finishing ability in the game, but he's not the best finisher in the game because he has has to play under the rim due to his lack of reach/athleticism.
When dissecting a player's abilities and projecting their ceiling, of course nothing is known for sure, but it's all about gauging probabilities by what we know about the league and what we know about players in basketball history. You have to do this because it's important to knowing how to build your team. Even with a LeBron James or Kobe Bryant, you have to build around what they do and the way they do it. You have to know what a player's strengths and weaknesses are (and it isn't always big overall categories,) and the likelihood of them improving on them, how much they can improve on them, and the way they can improve on them.
. yeah i said it. it sounds stupid but its true....
