To be honest, I didn't know that Coman and Nkunku were two players from the PSG academy, and it would be stupid for PSG to let go of the talented talents they once had, I think maybe there are still a lot of PSG academies that they have wasted at the moment, this is just to take a shortcut by spending a lot of money to recruit star players who may not be better than their current former academy players.
Nkunku is a pretty good player but at the PSG academy it seems like he is not very visible so it is quite natural that he let him go besides that the situation of PSG's midfield and attack line when Nkunku was there was also still fairly luxurious where several players like Di Maria, Icardi, Drexler and Neymar also made Nkunku's very young performance invisible because of strong competition and if in the end Nkunku remained at PSG we would not know whether he would be like now or not.
In Leipzig he got more comfort and that was the opening for him to pave the way to be recognized.
So was it a mistake? I don't think so because in this case everything has its own time and priorities, Nkunku is good but he was in 20109 outmatched by several star players so indeed the last hope is to leave rather than his talent being wasted when he becomes a player on the PSG bench.