I think you are right about the Osimhen situation, but Napoli seem more confident about keeping Osimhen than selling him. Osimhen can help Napoli to defend their title next season, but I doubt Napoli will repeat its success again without Osimhen.
In the end Napoli may need to accept another team's proposal for Osimhen if the player does not want to stay. I'm sure Osimhen won't be able to maintain the same form as last season, so Napoli should also consider selling him instead of having to apply for a longer contract extension. 120 million euros is a fantastic value, so Napoli has nothing to lose to release him to interested teams.
Then they have 120 million euros in the bank and then next? I think approving transfers for some of the top players should involve more than just the transfer fee. If a club knows that they won't be able to replace that player because there are no other players available, I think the situation changes quite a lot even when the transfer fee is tempting. But selling a player is only the first step. The gap must be closed and they can either risk it and hope that they find some player who performs amazingly but cost much less than the 120 million, or they could have issues and miss a striker badly in the next season. The market for strikers isn't easy right now and as soon as it is about one of those extremely strong strikers, 120 million might not even be enough to replace Osimhen.
For me this is good decision by Napoli and Osimhen to stay here for another season despite having few luxury offers from Al Hilal and other clubs because they need to keep this momentum and also having another try for the double which they missed in last season but now again they can go through for the which is surely good specially Osimhem which is young and have long career ahead he needs to keep his performance at top which is only possible for him to stay here and give his best which allow him for doing same performance for the next season.
Victor Osimhen already have tag of €150million because Napoli president insist they are not going to have any offer for this season and with this he personally wants to stay here for another impressive season which keep him in better shape and with this most chances he could be able to shift to his childhood club Chelsea which will also improve his quality in this season.
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.