Yes, I fully agree with you that Osimhen is a talented player, but I don't think management treated him unfairly. The truth is that Napoli was not in good form this season, and that is why we did not see Osimhen work much. As it always works, when a team is not in good form, As it always works, when a team is not in good form, it will seriously affect players performance, no matter how good the players may be, so I don't think that Napoli management mistreated Osimhen. If it is about discipline, the coach needs to discipline this player if he commits an offence, so to avoid unnecessary actions from repeating themselves. If Osimhen wants to leave Napoli, I will prefer him to join PSG over Chelsea because, currently, we don't know where their performances are ending.
The fact that osimhen is a talented player and have been able to achieve quiet a lot with them the previous season is. A reason he would should have been seen and appreciated differently even with some preference away from other players just like found in other leagues where they have got preference for some of their top players but then it was sad they didn't even consider osimhwn that much.
Leaving has always been what osimhen wanted, considering all that was still happening in the team aswell, he was not really getting to bond well with most of the team mates in the team this season, the team was also characterized with so much of internal crisis which aswell affected the team and their general performance was really poor and slow as you did mentioned because they weren't able to even get to get good results and they had to battle with consistency with which osimhen later suffered injury which aswell affected the team.
They have got a whole lot to fix this season of which if they are able to get it fixed before the end of the season again, rhey may be able to get better results by next see but then the absence of osimhen is definitely going to cause some difficulty in the team such that they may find it difficult to get productive just as they have struggled all these years before the arrival of osimhen who brought them the trophy.