Every time we sell a young player, we should lock down their threads for 3-4 years and then come back. Will avoid a lot of useless debate.
In the last few years, the only young player who made done decently well after leaving us is Bonucci. Of course, there is Balo but he is an exception and had to be sold irrespective of talent. And there are numerous others who have gone bad like Acquafresca, Krhin, .. So far I have no reason to believe that Coutinho, Poli and few others will give us any reason to regret this decision years down the line.