So it is only players that the like of mancity and psg clamour to sign that are good.So the likes of mancity and psg clamouring for a player is what shows that are good. lol at that reasoning of yours
If a player good enough for the Barcelona first team is available isn't it obvious that top teams would be wanting to buy them? Not just Man City and PSG, but Barca, Madrid, Man U, Chelsea etc would all be trying to get them. Of course if that's not happening, then it's likely because of genuine doubt as to the quality of the player or because the player isn't available.
In the case of Barca there's the added problem that the player needs to have the right profile to play in a high line tiki-taka defense. The players you mention would all likely be 4th choice if they came here. Even David Luiz might end up with the same problems Alex Song did. Unfortunately it's not that easy to get players and get them to fit into the team successfully.