Like any human-run system, these programs are flawed. You can undoubtedly make cases for every selection process that some deserving kids missed out.
From a top down level, if you are evaluating whether Victoria's SDP and NPP programs are successful, you'd have to start with their 2 primary goals. I think the lesser one is to identify and develop athletes to win future national championships for VIC. By this measure it's hard to argue that SDP is helping, or at least not hindering, VIC's dominance at nationals. They are picking enough of the right kids at SDP tryouts, to then select state teams from, to win year after year.
The greater goal, in my opinion, is to identify and develop future elite athletes for national team selection and professional careers. There have been some questionable selections here in retrospect - was Giddey really only good enough to make Vic Metro as a top age U18s player? Was Bogut really only good enough to make Vic Metro as a top age U18s emergency? You could argue that their special level of talent was not identified early enough. And I wouldn't disagree that there is a bit of "we've picked this kid before let's stick with him" mentality, that may have made it harder for the Giddey's and Bogut's to break into the system in their late teens rather than earlier. Of course I could just be off the mark and Giddey and Bogut just weren't good enough at their earlier ages, and the fact that they got snubbed for state team selections were essential into shaping them into the players they became.
A lot of the noise I observe from the kids, and parents of kids, that just miss out on SDP, is that they are better than some of the kids selected because they score more on Friday nights. I'd imagine the SDP selectors are considering a range of factors, scoring ability is important, but that's just one ability. So many Friday night top scorers barely pass, rebound, defend, etc. And their physical traits are eventually going to limit them in the game - they are just not tall enough, they are just not athletic enough, etc. To be brutally honest, I think most of the kids on the SDP fringe have a Big V ceiling anyway, so missing out on these programs really does not have much of an effect on the basketball world.