The main reason why many fans were sceptical about Dias was the unhealthy obsession with pace and Dias, unlike CBs like Kounde and Koulibaly, isn't the quickest CB out there. Dias' biggest quality is not a particular individual attribute as a defender but his ability to organise and encourage people around him. While Van Dijk has better physical attributes and is probably more difficult to beat in 1 vs 1 situations (but he is/was in his prime!), he can't quite organise the defence like Dias, that's priceless about Ruben. Baresi wasn't a (very quick) physical beast either but is rightly regarded as one of the best defenders ever.