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.
If you believe Conte will make Napoli better, but why do you assume if Osimhen stays at Napoli his market value will drop. Because anyway, if Osimhen stays at Napoli and Conte manages to make Napoli better, then of course there will be a big chance for Osimhen to experience a good lift as well in performance and also market value which will certainly improve as well. But anyway, it looks like Osimhen has also decided to leave and also, because Napoli will only focus on the domestic league next season then I am sure Conte will succeed in making Napoli compete in the top four of the Serie A standings next season. Therefore, although Osimhen will leave but I am sure Conte will still succeed in making Napoli stronger next season.