I don't watch Serie A but what happened to Osimhen? Did he just move on from Napoli in his head and so lose form? Or did he start to slow down?
He probably isn't as good as two years ago, but much better than last season. A big reason for what happened is that Napoli lost their manager who won them the Scudetto in Spalletti to the national team. That's when he was at the top of his game through Luca's direct offensively flowing tactics, which took everyone by storm. After Spalletti left, they thought they replaced him with a good hire in Garcia, but he turned out to be a major dud. Victor and Napoli had three managers at the end of last season, each with their own tactics and philosophy on football. Even though someone like Kvara outplayed Osimhen, Kvara also looked a lot more neutered than under Spalletti. When you have three different managers, nobody will find a rhythm.
What happened is that Napoli always had plans to sell him, they knew they would never keep him long-term and were hoping a wealthier club would come in and trigger his 130 million Euro release clause this summer as the next "Haaland" in terms of hype. However, because de Laurentiis failed to hire a good manager, all the players' forms dropped and so did their worth. Napoli overplayed their hand and now wants him to GTFO because they decided to build around Conte, who doesn't see Osimhen in his tactics. They also want him to GTFO because of the high wages he has had since signing his last contract.
He is still a great player, but again, not at 130 million Euros worth. He's probably worth around 80 million and should be a consistent 20+ goalscorer, even on a club like Chelsea.