You must not be reading my posts. Players are still growing at 18. They often put on 20 pounds in that first year out of high school. The difference in physicality and competition in general is big between high school and college and massive between high school and the NBA. The only thing Derrick Rose had "proved" at age 18 was he was better than just about every other 16-18 year old. In my opinion that doesn't entitle you to a spot on an NBA roster. Guys in high school face vastly varied levels of competition. Even if they dominate the AAU circuit, that still doesn't mean they are ready to dominate college, let alone succeed in the pros. Other sports like baseball will draft an 18 year old, but he still has to prove himself over and over against his peers over a period of years before he gets a spot on a pro roster or access to really big money. The NBA doesn't have a viable minor league and not enough roster spots to go around.
I'm reading your posts. I just don't understand why you're so concerned about the development of an athlete or the ways owners spend their money. If a team owner wants to spend his money on a risky, undeveloped prospect, they should be able to do so. Wayne Rooney was physically developed enough to be a Premier League star for Everton when he was 16. Lots of other players weren't, so Everton didn't sign them.
LeBron James would have been developed enough to play for the Cavs when he was 16. Lots of other players would not be.
If a team signs someone who isn't ready, then that was a bad investment and they'll just have to be patient and wait for the player to develop. There's no harm in that other than the player not getting the in-game minutes they would get playing at a lower-level, but at the same time they are training with better players, coaches, trainers etc and making a lot of money that could significantly improve their family's position in life.
Since we're all making bad analogies, does it make business sense to give every kid that made a 4.0 in high school a really good job? Or should you wait until they are surrounded by a lot of other smart people in college and see who stands out from that crowd?
If you run a business and you think you've found a high school student that is the best candidate to fill a position in your company, by all means, hire that person. If you're wrong, you failed because you made a bad evaluation of that person's value.