For me, this guy is even more important than Frenkie De Jong. Frenkie is a great midfielder, sure, but you can find several good alternatives in that department.
De Ligt on the other hand, is one of the most promising central defenders in the world, hands down. He will be a new Pique, Ramos or you name it. Together with Stones and Laporte, he would represent a top three defensive line in the world for at least 7 or 8 years. City's defense could take them to higher grounds in the Champions League too.
For me, De Ligt is like Mbappe in the defensive department. Look how he plays and controls the attackers already, and he is 19! This guy... I beg City to buy him. He is worth 80-100m. If not now, he will be.