Sakho has gone backwards - I remember when he broke through and everyone thought "here is the next Thuram" - seemed so composed at 18/19 too, but he never really pushed on, and PSG eventually dropped him even though they aren't averse to playing highly talented youngsters despite their oil cash (Veratti, Lucas Moura).
Maybe a move is what he needs, but for 23 million euros we were probably right in not being interested.