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.
Yes, he still has the opportunity to improve his performance, but it all depends on Napoli management who will bring in a truly experienced coach who can give confidence and improve the quality of the players' performance and of course Osimhen will be more reliable after the arrival of the same coach as Spalletti last season.
Seeing Napoli's current condition, it is impossible to sell Osimhen before being able to bring in an equivalent replacement player because Napoli really really needs Osimhen's contribution which can help the club to win the Scudetto title like the previous season.