How, exactly, could you argue that at this point in time?
I dont think there is much difference in quality between Matthijs de Ligt and Harry Maguire.
I do understand that this is a Manchester United forum and that Maguire is 1) captain, 2) English 3) played well recently and that i should
overrate Maguire as one of the best in the world but i really dont think Maguire is that much better (if better at all) than Matthijs de Ligt.
They did cost about the same, have comparable qualites in leadership, attacking headers and both are good defenders. However de Ligt is much younger, does have experience playing in the semi final CL, played the nations league final, played in the CL, won the golden boy award and part of last season FIFPro XI last year. Also de Ligt is capable of sprinting and turning without it looking awkward... something Maguire isnt capable off. I do think Maguire his lack of agility could cause some problems in the future. Not that de Ligt is the most agile defender in the world but he is more agile than Maguire. De Ligt is also much younger so i could see him become much better, defenders usually do get better with age.
De Ligt didnt start great at Juventus with all the hand balls but he has recovered of his bumpy start and is playing fairly well right now. He also had to play as a left central defender at the start of the season while he is used to playing right central defender. It might be a small thing but when you are that young and arrive in a new country, new club and have to play a totally different style than he was used to, it does make a difference.
Ofcourse you dont know how de Ligt would have done if he came to Manchester United in the state Manchester United was in during the better part of the season. However i do think de Ligt has the mentality and leadership qualities to have stepped up like Maguire has done recently. I am not saying that signing Maguire was a bad move because he is the best defender at the club but i doubt that signing de Ligt would have been a worse move. It might even have been a better move long term. It was never an option to sign de Ligt however since de Ligt never wanted to join Manchester United.