I do not think Napoli should sell Osimhen and buy Lukaku instead. Osimhen is better player and he is younger and more talented. Conte is great coach but getting rid of their best player does not make sense if they want to win Scudetto. Napoli needs Osimhen to succeed especially if they want to play in Champions League. I understand they want to strengthen their team but replacing Osimhen with Lukaku is not the answer. Napoli should build their team around Osimhen not replace him with someone who is not as good.
It looks like it will actually be a setback for Napoli and although last season Lukaku played quite well with Roma, I also can't guarantee that he will perform well at Napoli in the future. On the other hand, if Osimhen is willing to stay and listen to Antonio Conte's instructions then he can find the performance that was lost last season.
Napoli's long-term project can only be seen when Antonio Conte wants to become the new coach and other players, not only Osimhen, should understand that Antonio Conte is the figure they have been waiting for. They have to try it first and I'm sure they will be satisfied with the strategy that this Italian coach has.