I agree with you that Antonio Conte has great potential to bring positive change at Napoli if given full freedom to form the squad he wants. Of course, losing a key player like Osimhen would be a big loss for Napoli because apart from his young age, Osimhen also has great quality, especially since Osimhen was once the top scorer in Serie A.
However it is not easy for Antonio Conte because currently Napoli is in a slump and I think it will take time for Antonio Conte to revive Napoli's performance and it cannot be denied that Antonio Conte is a very experienced coach and he also has enough experience in Serie A so It is not difficult for Antonio Conte to adapt, but now the competition at the top of Serie A is getting fiercer with several top Serie A teams starting to rise so this makes it even more difficult for Antonio Conte with Napoli.
The news about Osimhen is uncertain because Napoli are trying to keep him and with Conte coming, of course it will be a consideration for Osimhen to stay with Napoli.
I don't expect Antonio Conte to bring Napoli back the title they lost and I don't think Napoli's owner is targeting that, being able to bring Napoli back to the top four is the best achievement that Conte will achieve in his first season with Napoli and with his experience it looks like he will be able to achieve by Conte, especially if the existing squad is retained.
There are rumors that Osimhen and Khvicha Kvaratskhelia will leave Napoli but the Napoli president has made it clear that he has no intention of selling Khvicha but for Osimhen it looks like he will indeed leave this club but if Napoli can keep him of course it would be better although that will depend on whether Conte really wants Osimhen in his squad later, because It could be that Conte has a different view of the striker.