In addition to Osimhen's price which could drop because he continues to insist on being kept, Napoli could also experience more defeats because there is one player who may no longer be happy to be in the Napoli squad, namely Osimhen. So Napoli must immediately make a wiser decision if they want their own team's condition to improve because losing badly without reply in the first match this season is certainly very painful for Napoli, especially if their opponent is not strong enough but Napoli is unable to score a single goal against Verona.
I don't really understand Napoli, even though many clubs want Osimhen, Napoli doesn't seem to want to sell him because they deliberately set a very high price so that many clubs don't want to buy him. And it's true as you said if it continues like this, it is not impossible that the price will continue to fall. Even though on the one hand Conte is not trying to keep him because he prefers Lukaku.
If Osimhen fails to perform exceptionally well this season, his values next season will fall drastically and not many team would want to buy him. This is the reason I would have wanted Napoli to sell him now that many clubs are interested in him. They are willing to offer significant amount and not the outrageous price that Napoli is setting. I would have prefer him joining Arsenal because he has brighter chance there as I see Arsenal doing very well this season far more than Chelsea that have no direction as of this moment.