Vinicius and Rodrygo were absolute certs barring injury. As good as Mastantuono looks, I've read from people that watch him regularly that there are a lot of fundamental things he needs to work on so for the price they are looking to sell him, that presents a big risk. It's been a very long time since the 'next Maradonna/Messi' coming from Argentina (Thiago Almada, Echeverri, Presstianni) lived up to their expectations past youth level so its fair to be cautious.
It would be silly that someone should invest in a youngster ONLY expecting a Messi or Maradona, neither is needed, how dumb we can be if we expect that level of player every year.
Even if the player being Argie, there are more possibilities. BTW It's not a long time ago that players with a very similar build to the 2 geniuses (pacey, short and bulky) had great talent and succeess like Kun or Tevez, or even recently less talented, yet similar bulky build a very good player like Alvarez is doing his thing, without needing to be such an off the charts player like Diego or Lio.
As a side note, lots of the time someone is named the next Maradona or Messi, it's just silly, and in this times has more to do with earning clicks than nothing else.
None of Almada, Prestianni (that people even in Argentina actually barely know) where trully touted the next Maradona or Messi, as much, as replacements of Messi for the future regarding the role or the next more talented young "10".
Almada is a strange case due to his path, going to MLS was bizarre. He is a talented player, nothing even remotely close to Diego, still with time to make a better path from now on, still quite young.
Echeverri is lightweight as it was Aimar and if he ends like him, it would be great. Mastantuono looks like Alonso (the Argie one) rebirth, not at all similar in style, size, build main characteristics to Messi or Diego and Prestianni looked like a short, pacey dribbler, yet never even gain lots of recognition and certainly didn't look at all like a cert since day one like Diego, Lio or even less talented mofos born cracks like Kun or Tevez.
So at the end of the day, the invest should be in terms of trying to buy a future a Elite or Top level player and the mentioned still can end having a carrer in such level, the future will tell.
PD: Regarding Franco, he should stay in River to the very least one more season, he is very green and the expectation and state of Man Utd right now might end fecking everything while he still is literally growing.