Yes I know Osimhen performed much better than Lukaku last season. Even in the 2022-23 season Osimhen performed brilliantly. Osimhen scored 26 goals and one assist in the 2022?23 season. And Lukaku scored only 10 goals. So in all statistics Osimhen is ahead of Lukaku. However, Conte's favorite player is Lukaku. Napoli management keep Lukaku in their squad. Napoli management was not interested in Osimhen because of his high wages.
However, we don't know if Lukaku can do well next season. However, he managed to score in the first match with Napoli. If this streak continues, it will certainly bode well for Napoli.
What you say is true, Osimhen is a diamond, a brilliant striker but he also has some flaws. For example, he is not capable of dribbling and needs a team that supports him and passes the ball to him in the right way. He is not the type of striker who looks for the ball or looks for opportunities.
So you have to know how to fit into your team.