Players level and performances don't grow exponentially.
Messi was 19 year old when he scored that hattrick in the Clasico, the most high profile football match in the World or at least Europe.
He was 22 when he won his first Ballon D'Or.
Even if Mbappe at 18 is better than Messi at 18 was, Messi at 22 is probably a better player than Mbappe ever is going to be, the odds are stacked against him. And that's not even close to the best version of Messi.
I think you might be going a bit over the top here. What Messi has achieved and Cristiano's statistical output and consistency is freakish. We might never see that again.
These guys scored 50 and 45 goals just in the league, and a competitive league at that.
Mbappe surely got the potential to become an ATG though. Ronaldo, El Fenomeno was a better player than Mbappe too. He was just magic on a complete different level, a different beast whose career was hampered by injuries.