I agree with you that Antonio Conte has great potential to bring positive change at Napoli if given full freedom to form the squad he wants. Of course, losing a key player like Osimhen would be a big loss for Napoli because apart from his young age, Osimhen also has great quality, especially since Osimhen was once the top scorer in Serie A.
No doubt, Osimhen is a good player. Osimhen is among the top 10 strikers in the world right now. But when a club gets a new coach, the first thing the coach will do is to assess his players to see the ones that will not fit into his tactic so that he can get a replacement immediately to enable him to perform well. Already, Conto wants to bring Lukaka to Napoli, which I think will make Osimhen want to leave the club to sign for another club where he will be a regular starter.
If Antonio Conte brings in Lukaku as a striker for next season to play for Napoli, that does not mean that Osimehn will be less useful. Osimehn stats is way much better than that of Lukaku who has come of age now and doesn’t really have much time left to play football before he retires. I know Osimehn will want to change club and get registered under a new team, but he’s still valuable to Napoli if he stays back. I haven’t heard Osimehn linked to any team so far the transfer window, are they no big clubs interested in this striker or his release clause is beyond measure to be affordable by teams that will be interested? Leaving Napoli is not bad but if he doesn’t get a better place than Napoli to play for with more or same playing time, he should just continue with Napoli.
I don't see Lukaka as a backup striker; if not, Conte would go for a younger striker who still has more playing years ahead of him so that when Osimehn leaves Napoli, the striker will replace him. But for Conte, choosing to go for Lukaka, who has fewer playing years to retire as a professional footballer, means that Lukaka's style of play fits into his coaching style. Big clubs might want to secure Osimehn's signature late so that they will not get many rivals, who will make the buying price of Osimehn so high. If any big club indicates interest in signing Osimehn now, you will see that other big clubs will be interested in Osimehn, and the competition will make Napoli want to sell Osimehn for a lot of money because he is hot in the transfer window.