I never said Castle's shot won't improve, of course it will. I'm just saying that I don't think it will ever get to league average levels.
And, yeah, there are many bad college shooters that became good NBA ones, but there a lot more than remained bad.
There are different types of bad young shooters. With Wemby everybody could see he would be good with time. Not only his form looked great, he never shy away from taking them. Then you have a case like Kawhi, who clearly fixed his form and became elite overnight. On the other side you have a guy like Sochan, that is easy to see why he's an awful shooter, and then there's Castle, whose from looks good enough, but his misses are consistently awful. You can see from the get go they have no chance of going in. Despite his seemingly OK form, there's something fundamentally broken there. I don't know if it's something technical or just mental, but that's what gives me the impression that he will never get to even a league average level, shooting wise.