Actually, it is not impossible if he becomes the best player even though he plays at Napoli, we know that in the past Napoli also produced many stars, but at the moment it is a little difficult because Napoli as a whole has experienced a decline which ultimately makes us doubt them. Last season they managed to win the Scudetto title but this season they are very different and have now been thrown out of competition at the top of the standings.
I agree with you that Osimhen should seek new experience in a new team that is in a more competitive league. There are several teams interested in him, but Napoli have set a price of 100 million. In my opinion, this is still quite feasible because Osimhem's performance is actually still good even though the team as a whole has experienced a decline. So far, I haven't heard of anyone making an official offer to Napoli, but according to rumors, there are at least 2 teams that want to bring Osimhen from Napoli for next season.
Regardless of the loopholes Osimhen faced for a few weeks with Napoli, he still found so much belief in them and he'sore or less the fans' favorite too. So the news of his contract extension was definitely soothing to the team and fans too. Now it's certain that they'd keep seeing and benefiting from his unique style of football. He's being a key man at the Italian side and it was although very much unlikely for them to let him slip off their arm. He's just got more impacts to make. Napoli themselves have seen the quality of player he is and they really don't want to let him go. It's his decision no doubt, but maybe he's just got to get experiences in a different club environment. Even though, he still has an achievement to deliver onto the team too.
Since Osimhen joined Napoli, he's been their key player and he's been doing his striking job perfectly without errors, even in times of emotional derailment from fans and the coach, he's still being able to keep his mind focused. He's in no wise the best thing that's happened to Napoli. He's the best addition to the team. Although Chelsea are having eyes on him but it's likely that Napoli won't let him slip without a very good replacement for him. His exit will place a threat to the teams performance subsequently and they may likely go back drastically.
For Chelsea, he may not be able to cope immediately, he'd need time to blend and it's likely that the Chelsea pattern may not fit him. No doubt he's got the decision to make but he's got to weigh the options too considering that Chelsea can as well give him a whole new experience to football even though they're declining currently in their performance.