Osimhen has become a burden for the team lately, he fell out with De Laurentiis and later with Conte, who suspended him from training, so for discipline in the team they had to get rid of him. Lukaku has fit in very well, you can see that he is trying very hard, he likes working with Conte and that was their original goal to get him in the team. If Napoli, Inter and Juventus keep up the same pace until the end of the championship, maybe we will have something like competition in Serie A.
Osimhen had too many problems that he decided to be loaned to Galatasaray with various speculations. The dispute with the coach was the reason why he no longer felt comfortable playing for Napoli, although there were other reasons that could have influenced it. Lukaku's arrival changed everything where Osimhen began to be forgotten and maybe Conte preferred Lukaku to Osimhen so that he got a place in the team.
Conte's presence is expected to re-energize Napoli players and they managed to become the top of the standings. Conte has really made a big change for Napoli so far even though they experienced a defeat against Hellas Verona in the opening match in Serie-A if I'm not mistaken.