The diff is that Cashley was one of the best Left Back when he moved to Chelsea and in other hand Cech, Luiz, Willian all were in their decline. Havertz was hated by Chelsea's fans so it makes sense that they had zero problems with this transfer.
For ex. If we try to buy their best player right now like Enzo or Palmer they will go nuts for sure.
Ofc he is a legend and In my opinion he is the best winger in EPL history.