You are right that Osimhen is not interested in extending his contract with Napoli. This is why Conte is not interested in Osimhen now. He wants to add Lukaku to the squad. Because Napoli management will try their best to sell Osimhen this season. By selling Osimhen, Napoli management will earn a huge amount of money.
Osimhen is talented. He has managed to score a lot of goals in the last two seasons. Several big clubs like PSG, Chelsea, Arsenal and Manchester United are interested in adding a talented player like Osimhen to the squad. However, 130 million euros is a very high price. No club is interested in spending so much money. If it is 75-100 million Europe then maybe Napoli management can sell Osimhen this season. But the Napoli management is not interested in selling Osimhen for less than 130 million euros. Osimhen's future is still completely uncertain.
It might be difficult to wait for another club to pay more than 100 million because it is too risky and seems unreasonable when players who are not yet star players are offered at very high prices and Osimhen can only hope that Napoli will reduce the price of releasing the player.
If later a club makes a big offer and Napoli still rejects it, perhaps Osimhen will become even more uncomfortable and will only perform as he is without maximizing his performance.