Neal sits on his hands while Ozuna could have been acquired. freeing up Marte. Watching the Cardinals work the winter (as an example) highlights just how unlikely the Pirates are to ever build a championship team. (and if the Cardinals get Donaldson....).
Why would he feel he needs to move them if they are highly evaluated? I don't blame him for not moving them, I blame his evaluation as a whole.
He has done this with new players too, not just his drafted prospects or international signings. Niese and Jaso come to mind. They weren't close as good compared to the players they replaced. Harrison over Walker too.
I do agree with you in not seeing that certain guys might not pan out, but how do you expect a return for those guys. Guys like Tony Sanchez, Alen Hanson, and Brad Lincoln come to mind. He probably held onto them too long. But what could he have really got for them too?
Tyler Glasnow might be that next guy. Do you trade him now or hope he is a fixture for the Pirates? That is always a tough call. I am also watching all the middle infielders the Pirates have who should be coming up. I feel sometimes too many is a bad thing too. He should pick between Kevin Newman, Cole Tucker, and Kevin Kramer and maybe deal one of those guys.
I don't know why Huntington has never traded a top guy. He has made some really good deadline deals, but didn't have to give up anyone of significance in return. I don't know why he has never made an off season trade involving prospects. I feel he has had plenty of opportunity the past six years or so.
We're saying the same thing I think

It's not that he should trade his best players, its that he has not evaluated talent well enough to know who is going to live up to potential. I would think internal valuations should be much better than another team's scouting. Polanco and Pedro could have been traded for a king's ransom early in their careers. Was there really nothing for the Pirates to "see" on those guys? Polanco's big swing and shaky defense - nobody ever said "hey, he has some holes that makes him riskier than the book says. See what you can get for them".