IMHO - a few opinions:
0. Sounds like a few disgruntled parents (again)?
1. There is not enough money in Australian NBL/WNBL to have a youth league.
2. Being good as Junior, especially in Basketball, is unequivocally no guarantee that the player will make US College, State, SEABL, NBL/WNBL, Euro/NBA! Unfortunately that is the nature of Junior sport. Kids who mature early have an undeniable strength & speed advantage. For the boys, it doesn't start to even out until top age 18's or U20's. There are many cases of kids being Centre's in U16's and by U18's are only guard size. Needless to say many haven't worked on guard skills, haven't played as a guard and probably wouldn't have made a state team if they were a guard in U16 - as they're not good enough. They only made the team because (in general) they were big and strong.
2. I seriously question that the majority of kids that go to the AFL would have ever been good enough to make it in the NBL. Some may make state U16 Basketball teams or been on the cusp of selection, but most that I have seen over the last 9-10 years were probably never going to make state U18 or U20 teams for a variety of reasons (not big enough, not athletic enough, not a good enough shooter/finisher, not a good enough player, didn't have the right work ethic, had skill deficiencies that were never addressed, etc).
3. Do your research - how many kids who have made State U16 or U18 teams just fall off the basketball lanscape? I could name many that are not playing in State, SEABL, Big V leagues. They're just not good enough. Then there are the players like Kyle Adnam (Melb United) who never made a Junior State team. Why? My guess is because, for a basketball player, he was physically small/slight and still is. He didn't fit "the mould". I remember at State Talent ID camps coaches asking kids "How tall are your parents". I heard Andrew Bogut didn't make a State U18 team - because the coach/s didn't like his attitude.
Fact - The 2011 a National Championship winning U16 boy's team (Vic Metro) only had two players in the state U18 team two years later. I am sure there are many cases of this happening. Why? Development, maturity, other players working harder at their game, etc.
4. The NBL pool of teams is smaller now that it has been - this directly equates to a loss of roster spots. So 'back in the day' the likes of a Joe Ingles could go play NBL straight out of Juniors and have an impact. Who could do that now? Yes we've had the Ben Simmons/Dante Exum. But players like this are few and far between.
5. There are many players that have played Div 1 college that just aren't good enough for the NBL, let alone the State/SEABL leagues.
6. The NBL don't have limitless financial resources to uncover potential talent. The NBL clubs all keep an eye on the National Junior Champs and what's happening in US College. They have DP spots for up and coming players. So it is up to the player/s to 'earn a spot'. Game time is not going to be gifted and nor should it. The school of hard knocks.
7. Most NBL clubs are well connected in the Australian Basketball fraternity and know who is around. Good word travels.
8. Finally - how many kids have gone to the AIS (COE) over the last 20 years? Of those - how many have gone on to have a "reasonably" successful and lengthy professional careers (NBL, Europe, /USA)? My guess is less than half.