I think the reason why most Saudi Pro League clubs started with signing veteran players in the past was because these players were big names in football and were globally recognized so in their attempt to make the league popular among football fans across the globe, they were going for these veteran and popular footballers instead of signing young talents that will not attract global audience to the league. To an extent, they've been able to achieve their aim of attracting global attention to football in the Middle East country which is why they are now signing young talents.
Signing Ângelo Gabriel was a good transfer deal for Al Nassr even when I think they needed to sign quality defender instead of a winger. Hopefully for the Brazilian winger, he succeeds in his new club.