Hard to say who is better or worse as everything Juve touches turns to shit at the moment (and for ~3 years now). We'll only find out if Juve recovers or after they leave.
- - - Updated - - -
De Ligt had more than just a few bad performances. Defense has been utter shit ever since he joined. I am not saying its his fault, defending has to happen as a cohesive unit and is very tied to the midfield's performance as well, so its hard to say its just him. The defense is as strong as the weakest link and then everyone looks bad. Having said that, there's definitely a huge question mark over his quality because he did not thrive in his national team either the last few years. Cant really say he has been our best defender and thats considering the other defenders are Danilo as CB, Bonucci (not the Italy version), a constantly injured Chiellini, Rugani and the likes.
Dyabal had good seasons but these were from a long time ago. Time will tell if he truly was finished or not.
Overall agree with you that adding more talent is better than replacing one with the other but there are financial constraints.