I don't believe Lukaku is a better player than Osimhen currently. Osimhen gives you more than just goals, he plays with the team. He joins in the build up of the attack, he's also a hard worker. Lukaku was part of the reason Belgium got eliminated in the Euros. He waste too any balls. Any ball that gets to his feet gets wasted. I agree he knows how to score goals, but if he's not in a goal scoring position he can't do anything with the ball.
Lukaku is older than Osimhen and he has big weakness in his playing style and skills. These weaknesses, lack of ability to contribute to the team tactic when he does not receive good support, pass, assist from teammates are one of reasons why he failed in Manchester United and Chelsea. Osimhen is younger, faster and able to contribute more for general operation of a team. The Nigerian striker is a better investment for future than Lukaku who only can meet short term requirements like 2 or 3 more seasons.
Osimhen is a player who will progress, and has many seasons ahead of him, and Lukaku will most likely soon think about ending his career, or about a transfer to Saudi Arabia, by the way, he speaks very flatteringly about this champion. Therefore, exchanging Osimhen for Lukaku would not be the best decision for Napoli, unless they could sell Osimhen for the money they want to get for him, and buy Lukaku for much less money, because as we can see, Lukaku's market value is not that high now.
They can sell Osimhen and use part of that money to buy Lukaku or another striker. I agree that it is not smart to sell Osimhen to Chelsea and add Lukaku into the deal as an exchange player. Chelsea can want to do this deal that helps them to save transfer cost but Napoli won't want this one. They know that Osimhen can be sold at better price to other clubs and if don't have Lukaku, they can aim at other strikers as replacements for Osimhen.