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.
Osimhen was a great player who used all his efforts to get his place at the club and his performance was one which was praiseworthy only that I feel his efforts were not well banked on by the management as they did not do well to glorify his efforts much more that they should. What I can encourage the Napoli management to do this time around is to fully recognize him for what he's made off and should also work towards strengthening their team work and squad depth.