I understand that Napoli would want to keep their star players together for another season, but if the player is open to move and his release clause is activated, I don't see why Napoli would want to stand in his way after what he has done for them. I hope he understands that occasionally ideal moves do not pan out as planned; I would personally like him to stay in Napoli for another season.
If a release clause is part of the contract, there isn't any chance for Napoli to keep the player after unilateral deciding so.
Now I am not a friend of this "what the player has done for the club". You could also argue the other way around and the fans that carried him, gave him energy to score more and more. That's really not a valid argument. But I see a different problem and that is when these top players want something, they get it by force. They are injured, have mental problems out of a sudden, don't come to the training sessions, literally sabotage the team cohesion and so on and so forth.
If Osimhen can get the double the salary somewhere else and go to a top club, it is not unlikely that that will happen. Look how much Mbappe was able to push PSG financially. That is outrageous after he seemed to have given his word to Real Madrid already, but then took the 200 million in cash for signing the new contract.
Lewandowski same thing, or Ronaldo at Manchester United. Does anyone really believe that he wasn't talking with the Saudis for quite some time already before he gave the interview? In any case, the player has more leverage over the club than vice versa.