I really like Diop, I do, but he is 22, investing 60-70m is a huge risk. For years we have been trying to buy up and coming centre backs who might become top class, and we have failed every single time (including Lindelof). The other players people mention are similar Konate, Tah, Dias are all young players who are going to cost 50m, with the same risk of becoming the next Smalling/Bailly/Lindelof. We can't afford to blood them in gradually if we are paying 50-60m so we would need to have them come in as starter, which is difficult given most players have to come in and adjust esp if they are young and from a different league.
The really top quality players are unavailable to us at the moment, so the obvious choice might have been to get Alderweireld and wait a year. However that doesn't seem to be an option we are going to risk, truth is with the way prices have gone we could wait a year and find that Maguire is 85m and a Varane would be 150m (if he isn't that already).