I think people are under-estimating how hard it is to be a 'Xavi-esque' player in Brazil. Don't think I've ever actually seen a player emulate that in any South American competitions since I've been watching football. If you ever watch Brazilian or other South American teams play usually the offense is pretty stagnant and one dimensional in comparison to European teams.
A 'Xavi' is by nature highly dependent on the movement and spacing of his teammates and for Arthur to be a pretty effective orchestrating player in that setting is impressive IMO. He's not a surefire hit or a surefire bust, but he certainly has the right skillset to be a success. Instinctive composure on the ball, huge passing range, an agile ball carrier that retains possession well, a capable defensive support player. There's far more to like than to be doubtful about.
And he did it all winning the toughest competition in South America while being the best player. This doesn't make it a foregone conclusion that he'd be a star in Europe but you have to see that in his situation he's done everything we could ever ask of him to show and succeeded at the highest level. He could potentially not show it in Europe but he's shown everything he possibly can in Brazil, there's nothing lacking with the resources and stage given to him.
Edit: Saying things like "10m or pass" is actually crazy and I dunno what people have been paying attention to in this recent market to pull out a number like that. We've legit paid near that much for B team players, let alone a 21 year old transfer for the first team who has been one of the best players in SA, the Libertadores Champion, and MOTM of the final. I get the hesitation at 50m but srsly, no one should be all that uneasy or surprised that he's a minimum of at least 30m.