Re the whole "World Class" argument, I think in general there are just less World Class players operating these days now, owing to the game becoming more and more tactical and regimented
Often now it's about finding players that fit systems, rather than adapting systems to fit players
Barcelona are a unique case where both the calibre of the player and the system were of equal importance because so much of the team came from the youth ranks, but if you look at teams like Chelsea under Mourinho and Conte, a manager will have a preferred tactical approach and he'll promote and champion players who suit the style, meaning you can be an important cog in a hugely successful team without having to be "World Class" when it comes to technical ability
Look at Nicky Butt at Man United, no one would ever call him World Class but he was a core part of a very successful side because he did the "water carrying" for the other players in the squad, so that Giggs, Beckham, Cantona etc could provide the thrills
So this Koulibaly fellow may or may not be World Class, but ultimately that's less important than whether he could thrive in whatever system Koeman is planning to employ next season. So essentially whether he's World Class or not is ultimately a useless point of discussion that has no real baring on the relevant area of this debate, which is whether he can do the job for Everton that we'd need him to do. He can be World Class but be a round peg in a square hole (Thus what's the point?) or he could be below World Class but perfect for Koeman's system and thus entirely worth the money