IMO, a major proponent of the entire ordeal with "amateur" basketball players is the discussion on who's willing to pay for them:
The NBA doesn't make money off players until they're in the league, so they're not entitled to pay them
"At-will" employment for a basketball player around college is either college/G-League (which began recently offering competitive payments)/overseas - I suppose the NCAA banks on the reality that: exposure is limited overseas, and; the G-League only recently began offering competitive salaries.
For players, intrinsically maybe, you benefit from the exposure, playing time, and alumni (if you manage to leverage it as many players ultimately do). Sometimes, you can find value beyond the dollars and cents.
Now should they be compensated more, yes, but if I'm an old end-of-the-bench player in the NBA, no fucking way am I approving that: why would I potentially give up my spot on a roster for more younger (generally, and I suspect the NBA would renegotiate rookie salaries for players if the 18-year-old-draft rule was approved) and potentially more promising player: that even pressures middle-aged players who haven't panned out yet. The competition would be good for fans but if both sides had to initially disapprove it, that's an argument that makes sense for the players. The ownership, well that's probably pretty obvious.
