IMO 15-16 yo is too young to determine whether anyone will be a RM or Juve level player or not.
Robinho was Pele's heir at 15, Pellegri went to Monaco for 21m aged 16 (has to be some sort of record in Italy), Odegaard was a top top talent at around the same age and is nowhere near top talent at 23, our very own Moist Kean, etc. You're right that RM are often the ones who like to make these signings, but it rarely pans out for them, their best purchases out of South America are a bit older players like Marcelo and Casemiro