But the current restriction, and the want to raise it, isn't really about "student-athletes." It's about a league that prefers a free farm system to augment the problematic D-League.
That wouldn't make a higher age limit right, though. Just because the NBA might wish to funnel more players into the NCAA's free labor system doesn't make such a move morally justifiable. Why should anyone encourage a profession to further discriminate on the basis of age? Keep in mind, age 18 is when an American citizen legally becomes an adult.
Again, there is no correlation between "staying in school" and having a better NBA career. With that in mind, raising the limit might adversely impact pro development in this country. So many great players hail directly from the high school ranks—Kobe, KG, and LeBron to name a few—and they seem no worse for wear. The league should be looking into scaling the measure back, not fueling the one-and-done farce with an obstacle.