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.
I don't think Osimhen is any burden to the club. As a matter of fact, he has contributed immensely to the development of the club to the level that he was meant to be awarded a the king of Napoli, in my opinion, but the position went to Kvarat. I know the player was not in good times with the manager and they're misunderstanding relationship greatly affected the performance of the club in the last season compared to when Osimhen was still very functional
Going to Galatasaray was a break for him, but I wish he had go to Chelsea instead. Chelsea was a good option to Osimhen since that will affect his career strongly. However, Galatasaray isn't a bad idea yet and they need to keep the guy's spirit high to get maximum resultd from him. Football is all about emotions after you've taken out the skills and talents