If they sell de Ligt for a huge sum of money like 90-100 million euros then I don't think they would have any problem on signing a replacement for him. There is nothing more than defenders in the transfer market and there are many skilled ones also. Let's say they couldn't sign any then their defense would be in a bigger threat. Because I believe they would definitely need a defender right beside Bonucci to have a solid defense. Chiellini's departure makes them become in need of a solid defender.
Juventus could always get someone instead. Wish they didn2t sell Melih this year, if they didn't sell him, he looks like a young defensive star with a great potential, they made 20 million from that sale and that made sense at the time when they sold him. However, if they sell De Ligt, then it would have been worse, he should have been in the team so that when they sold De Ligt it would have been better.
This is why I believe that they will still sell him because it is a lot of money, and then spend like 20 million to get someone instead, would make a ton more sense. As a team who doesn't have "that" kind of money, 100 million or close levels would be incredible for them.