Napoli's management really understands that Osimhem is worth keeping for next season, it is impossible to get players at his level in the transfer market this time. If released, of course, Osimhen's replacement will need time and support so that he can be at his level at least one or two seasons for sure. If Napoli let Osimhen go for 150 Million Euro it is very reasonable, I agree with your opinion because Osimhen is fully tied to them for the next two years.
Napoli understand very well the current transfer situation if a club dares to bid at the price they want then Osimhen will be released. From the other side, the reason why Napoli maintain Osimhen next season i think they want to defend the Scudetto title, it will be very difficult to achieve it without a solid squad and sharp strikers. Holding him for one more season certainly won't affect Osimhen's price considering that he is currently only 24 years old.
Current transfer market from Victor Osimhen possibilities raised above 150 million euro but I don't sure if Napoli will get the same values if extend him one year later. But has chance for Osimhen values up drastically depend on have top performance or underperformance next season, I think better for Napoli include Osimhen in the name players list for selling in this summer because 150 million euro is most cheapest as transfer fees.
Napoli looks calm down and seems tries keep extend one year later for Osimhen, not looking for new replacement for Osimhen in this summer window transfer because Napoli sure with Osimhen will stay one year for defending Napoli with scudetto trophy next season.