Just because MKG hasn't improved his shot doesn't mean he can't when he gets to the NBA. Just look at Rondo, coming out of college his major weakness was shooting. He isn't a great shooter but he certainly is a lot better than he was coming out of college. Westbrook had the same complaint even though his stroke wasn't that bad. Magic Johnson was not a great outside shooter either but he improved a lot once he got into the NBA. Karl Malone improved his FT and added his fade away jumper to his game. There are several examples of players with unorthodox shots that were able to be decent scorers.(Barbosa, Marion, Pippen, Jamal Wilkes and Magic). Will MKG lead the league in scoring? Absolutely not but can he be a player who averages 17 or 18 pts a game. Yea I think that is possible.
You say that MKG has known since high school his jump shot was an issue. Your conclusion is that he should have already fixed it. Really? Are all players finished products coming out of high school? How much time does he have to actually train while he is in high school (being a kid and playing tournaments all summer) and one year of college (being a student)? When players enter the NBA they have a lot more time to work on their game besides having access to better training. Will MKG ever be a great shooter? I think it is too early to tell but with his work ethic and passion for the game I think the probability is very high that he can improve his shooting enough to be a great player. I think his greatness will be in filling up the box score and not just being a scorer.