Napoli is also aware that when Osimhen plays another good season, there isn't really any risk involved when they decide to sell him next season. They would still make a lot of money from that transfer unless he gets injured or his performance is really bad, but I guess he will stay a solid season. It will still be about the team effort and not so much about individual performances Napoli played so well because of the team cohesion and team spirit. Otherwise you can't play more than half of the season and only lose a single game. That was an incredible team effort and that one is tough to be repeated.
Osimhen is still young and his quality is also very good so it is possible for Osimhen to remain in top form, unless he gets injured (that's something that can't be predicted)
I see that actually Napoli really want to keep Osimhen aka from that they charge a very high price for a team that wants to sign Osimhen, that's something that seems to me as if they want to hold it, even if there is a team that wants to pay a very high price it means Napoli can get big profits.
But now we don't only see players from their selling points, but their performance and also their contribution to the team and Osimhen has the possibility to be able to make a lot of contributions to Osimhen, he is young and also has good quality.