Still I guess this transferring players at early age thing would be seen for only average level of players. I wouldn't expect any top player at young age to say yes to such teams. Because it wouldn't be good for their career no matter how much money they are offered.
Playing competitive for the Champions League title is the dream for most of such players in the end.
Some players may value their playing time and the money that comes with signing and playing football for such teams more than others.
If every player who joins the Saudi Arabia league does so for the sake of money rather than for the love of the game, the league will be unable to raise its standing among other leagues in the long run. If they want to become a globally competitive league, they must pay attention to potential young players when scouting for good players to play in their league.
I don't see anything wrong with Ruben Neves joining Al-Hilal; perhaps he'll get more playing time and, as a result, improve his career there. People go where they are valued and where they can make sufficient income before retiring. Don't be surprised; there must be a tangible reason for his action.