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.
Osimhen probably doesn't want to be at Napoli anymore because Napoli don't play in the Champions League competition, I think that's a natural thing for a player like Osimhen, it's appropriate for him to go to a much stronger team.
Meanwhile, NApoli's plan to recruit Lukaku cannot be separated from Antonio Conte's wishes because Conte is quite happy with Lukaku and I think from what has happened in the past Lukaku's quality will suit Conte's coaching style quite well.
Even though Lukaku's performance has really declined at the moment, I think every coach has his own point of view and we have to respect that.