When the complaint was the high number of busts among high schoolers, it seems to make sense to simply stop drafting them. Especially since they decided to put in a rule that says just that.
To your final point: Again I bring up Jordan, Magic, and Bird (along with all the other NBA legends who made a name for themselves in college for more than one year). There was never a rule stopping players like them from entering out of high school or after their freshman years. Yet they stayed. Are you suggesting that those players weren't ready for the NBA out of high school? Perhaps they weren't (Bird would have struggled from a maturity standpoint, from what he has said). But that also didn't stop them from becoming three of the five best players to ever play the game.
The main NBA complaint about high schoolers was that they didn't want to have to waste time scouting high school players. The one and done rule makes it much easier to evaluate players, especially big men, against better competition and also gives the NBA tons of free marketing.
And you really shouldn't use Magic to try and make your point. Magic went pro as a sophomore when it was unheard of for sophomores to go pro. He was one of the ones leaving as soon as he could. That was also a much different time when almost no one went pro early, Magic was much closer to a KG or Kobe, someone bucking the system and going pro early, than he was a four year college type.