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!