Yamal is not only Black African but Magrebian too so I don't think his ceiling is predictable for that reason, since Magrebians had some real talents like Zidane, Benzema (given not all Magrebians are pure Berbers as those two). Messi level there is no way, only one guy was close and he was Argentine. If you show me a black Iniesta I will start to believe that there is a possibility. Black Africans in a big percentage have a different body type. Their abilities are more akin to Di Maria or Haland than to Iniesta or Messi . The only close call to that profile is Mbappe and he is half magrebian too.
Now considering Yamal's actions I would say he can be like Henry or Cr7 with lots of training. So I do trust our boy to be a huge player but let's be real he isn't 50% of Messi in terms of talent. Messi is just 1 in 100 years phenomenon and genetically only Argentina produced a Messi like player. Given his stats while not being a total sociopath like CR7 it will be very unlikely to ever have a Messi in our shirt.
We got lucky then, and given that our fanbase is plastic that craves for UCL results not for buying talents i don't see it in the future.
(That's why the board busted money on Coutinho, Dembele, Griezmann, Lewandowski, Ferran, Raphinha and not on real Brazilian or Argentine talents who need 3-4 years to settle in).
Probably a Argentine will came close enough as talent in the next 50 years but Messi was just insane, and is very unlikely we will play him.
To conclude our boy should never compare himself to Messi, he should compare himself in training and style to CR7. He can emulate his style.