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.
Antonio Conte is a great coach and I'm sure Napoli will be even better next season. It is very unfortunate because Napoli only managed to finish in 10th position in the standings after they won the league the previous season. There are many problems at the club but sacking the coach and replacing him with Conte is a good choice in my opinion.
Osimhen is still young and has a lot of potential. Currently he has the best value on the market. If Osimhen doesn't leave now and chooses to join Napoli, I think his market value will fall over time and depending on Napoli's performance next season.