Player transfers are very normal and not something a mistake, because basically every player has the right to determine his career. Except, if for example Osimhen had chosen to always play at Napoli but Napoli sold him, then that case would be different. But anyway, basically Osimhen has been loyal to Napoli because Osimhen still chose to stay at Napoli this season. Because afterall, in the last transfer window it was also rumored that Osimhen had received an offer from Real Madrid and several offers from other big teams, but in fact Osimhen chose to stay at Napoli this season. But unfortunately, the results that Osimhen got this season with Napoli were not good and as such, it is very natural that in the end Osimhen chose to leave and that was not a mistake.
Yeah Osimhen prefers to stay and extend his contract with Napoli this means that Osimhen cannot make a decision in the next few seasons, unless Napoli wants Osimhen released, as you said, Osimhen's performance has not been optimal this season so his interest and market value will definitely decrease so there is a possibility that we will see Osimhen play longer in Serie A. This season Osimhen only scored 12 goals and this is proof that its performance is declining but Osimhen is still young he still has a chance to improve his performance next season.
Osimhen's transfer is still just a rumor. So nothing can be completely trusted regarding this rumor. The reason is, Osimhen once brought Napoli to become Serie A champions and won the Scudetto last season. So I think Osimhen will definitely not think about moving to another team for now. Although Osimhen once said that one day he also wanted to play in the Premier League. But I don't think that will happen next season. Because what is clear is that Osimhen has signed a new contract with Napoli until 2026. So there is no doubt, this means that Osimhen will stay at Napoli next season.
Even though Osimhen's performance is currently in poor condition at Napoli. However, there are many factors that make Osimhen's performance less good this season. Such as the lack of cohesiveness of the Napoli players, or the lack of chemistry between the coach and players. So in my opinion, Osimhen's poor performance this season is influenced by the poor performance of the team and coach.
So it is certain that Napoli can improve their bad performance. Maybe next season, Napoli must be able to buy new players who can cover Napoli's shortcomings or also have to replace a better coach. Because after Napoli changed coaches, Napoli's performance, including Osimhen's, dropped drastically. This means that the current coach is not suitable to train Napoli players.