By selling Osimhen, Napoli management wants to earn big amount of money. And that's probably why Conte isn't showing interest in Osimhen right now. Napoli won't have to spend a fortune on Lukaku. Moreover, Lukaku is a very talented player. If Conte uses Lukaku properly, Lukaku will be able to score a lot of goals.
Victor Oshimen has a fairly expensive selling price before his performance continues to decline and his price continues to decrease, the management wants to immediately release this player so that he can get a big profit and can buy player at a cheaper price. It is known that Victor Oshimen's current price is around 100 million euros, while Romelu's price Lukaku is around 40 million euros so the management can secure some profits to recruit more players if needed.
In my opinion, Lukaku's performance is also quite good. This player seems suited to playing in Serie A rather than in the EPL. In fact, he is the player who has contributed the most to Roma. If he was at Napoli, he could replace Oshimen's role if this player was really sold by the club.
Apart from the fact that Napoli wants to make some profits from selling Victor Osimhen but they should also know that Osimhen helped them to win the Scudetto in the 2022/2023 season and he's currently the club's most valuable player of which he deserves to be an expensive signing but since no club have been able to meet the demand of Napoli to sign him, Osimhen should still remain with the club.
As for Lukaku, i don't think he can play the role of Victor Osimhen because we have seen him play for Inter Milan on loan in 2022 and also moved to Roma on loan in 2023 and I believe that if he has made good contributions to those teams one among them would have considered signing him to their team. I'm not saying that he is not a good player, he is but he's just an average player and cannot be too relied on to deliver for Napoli. However, he can't handle the role of Victor Osimhen.