Sure he does have huge potential and talent even at age 18,19. But he didn't really achieve much at that age, due to his young age, inexperience, injuries, not yet fully developed as world class playmaker, lack of end products, lack of impact in winning trophies, better players ahead of him limiting his start etc, whatever, you named it.
Truth is, I always rate young Messi as one of biggest young talent I've ever seen, more so than Mbappe. even at 18, 19, I do think he is already better dribbler than Ronaldinho, who is best known for his amazing dribbling skills and tricks. But truth is, he hasn't done much at that age yet, and the bottom line is, football is competitive game, not a what-if imaginary game judged by glimpse of talent or who's having higher potential. Mbappe is simply better than Messi at that age, for what he has done on the pitch, and for his overall contributions to his team on winning games and trophies, despite being less talent and lower potential. But Messi is far better from 21-22 onwards, no questions about that. Only a blind or self-insecure person would be afraid to acknowledge the whole picture of that.