Conte wants to sign Lukaka, and when a new coach wants to sign a player in a position where he already has a player who is doing well, that means the coach does not believe in that player. Currently, Osimehn is better than Lukaka, but I don't know why Conte wants to bring Lukaka to Napoli. If Napoli successfully signs Lukaka for Conte, sooner or later Osimehn will not be happy at Napoli because his playing time will be limited, and he will not want to be in such a situation. All new coaches always want to sign the players that will make their tactics work for them so that their contracts will be safe. Osimehn is a good player that so many big clubs will want to sign if they notice he is currently not happy at Napoli.
I do not think that it is not about believing in a player, but more like believing he will stay. I agree with others that Osimhen will leave, that is why it is Lukaku, if you asked Conte which one he would prefer, I bet you that he would prefer Osimhen. The thing about building a team from ground up, and that is what he is doing because no matter if they won a year ago, this year Inter won and Napoli sucked, so it means Napoli isn't good at all this year, and that is why I think it is quite clear that he is going to have a lot of job.
Summer is not to get better, summer is to build a team, and they are going to build a team, both by selling some players, and also getting some players, after the team is guaranteed and roster is set, it will be easier for him to manage that team.