Here's the real issue:
Ballin's Law of NBA Improvement
"A player shall not play at a level significantly above what they have thus demonstrated in their first four years as a player in the NBA."
Go to basketball-reference.com and check out the stats, and tell me if I'm wrong. Some notes:
1. Those that are high on Rondo Jeff Green had best observe this rule.
2. Ballin's Law applies to FT% as well as everything else.
3. Steve Nash isn't human, so this doesn't apply to him.
There's stuff you can teach and stuff you can't teach. Athleticism, (to some extent) court vision, strength among other things can't really be taught.
A reliable jumper, which is what both of them need, and a post game, which Jeff Green needs, is definitely teachable.
Eh. I think you can get more athletic (while still in your prime) and stronger. Can Eddie Curry become Ben Wallace (c. 2004)? Doubtful. Can Durant turn his body into LeBron's? Not if you gave him a decade. But hard work, especially in the offseason, can make a player faster and stronger while more agile with better endurance. Court vision is one of the harder things to obtain if you don't possess it by the time you the NBA.
Jason Kidd is the notorious example of a poor shooter becoming respectable enough to open up the rest of his game. Is he a knock-down shooter? No, but he can keep you honest. The Mavs purposely let Rondo take a potential game-winning three last season. Teams (and the President, apparently) know he can't shoot, and now 5 years into his NBA career, little has changed. Is he better than he was as a rookie? Not significantly. In past seasons, our offense could overcome that because we had players who could score on their own. Now with the Big Three aging, Pierce is the only reliable option, and even he can only do so in spurts. He can carry the team to a W on any given night, but can he still go shot-for-shot with LeBron over 7 games? Not by himself.
Granted, this is the only true flaw to his game, but let's be real, it's an extremely important aspect of the game. From what I recall, he's worked diligently each summer with shooting coaches, most notably Mark Price. His form is still poor and has too slow of a release. Jumpers have a lot of variables, so even the best shooters can have good and bad seasons depending on the looks/defense they see throughout the year. But Free Throws are a pretty good indicator of shooting, and he is awful from the line.
If he doesn't make a large stride this season, he likely never will.
As for JG, he has a reliable post game IMO, he just needs to go to it more often, particularly when he is at SF. His jumper is relatively close to reliable, especially from his spots (corner 3's, elbows). He can definitely sure up his weak spots, but I don't exactly cringe when he hoists up a shot like I do with Rondo