there's only 3-4 players in history who were absolute beasts at the age of 20. messi, maradona, ronaldo, and pele. you can't measure dembele to any of these 4 as he simply isn't going to be on that level.
When he was purchased for that price, of course you can (and should) measure him against great players. If he simply isn't on that level, why pay that price?