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.
I don't think Conte has a long-term plan for the Napoli squad right now. Skoda would have tried to add young players like Osimhen if they had planned for the long term. Conte wants to keep the performance of the Napoli team stable in the next season. Lukaku's performance with Roma was quite good. Moreover, next season Napoli will probably be aiming to be in the top four of the table. Napoli do not have a strong squad to win the title.
This is the plan and I must say that it is an excellent plan, however there are some unforeseen events that are not being taken into account.
AC Milan have also probably made offers for Romero Lukaku.
This could mix things up a little and put everyone on a nice hot pan.
The Chelsea's pan!
Conte himself has a contract at Napoli until 2027 and as such, I think Conte must still have a young squad that will remain intact during his coaching period at Napoli. After all, having Lukaku or Osimhen in the squad will also provide good results for the team, and Osimhen is also capable of becoming a top scorer if Napoli have the right coach. But, about the benefits that will be obtained if Napoli sell Osimhen, I personally am not so sure about such reasons. Because anyway, Napoli also previously extended Osimhen's contract and if Napoli wanted to make a profit from the sale of Osimhen, then Napoli could do it before and not extend his contract too.