Yes, Napoli would be better off selling Osimhen to Chelsea at a fairly high price, and buying Lukaku in exchange for Osimhen's departure. The reason is that Osimhen seems to lack enthusiasm to continue with Napoli, this can be seen from his unsatisfactory performance this season with Napoli. Apart from that, if I'm not mistaken, Osimhen was also involved in a conflict with Napoli, because he felt ridiculed after failing to kick a penalty when playing against Bologna. This of course means that Osimhen and Napoli are no longer in harmony. Maybe that's why his performance this season could be said to be less than good with Napoli. So selling Osimhen to Chelsea could be said to be quite the right action for Napoli.

And recruiting Lukaku is also the right thing. The reason is, Lukaku's performance as a striker to date can be said to be quite good even though he is no longer young. Apart from that, Lukaku also has quite a long experience playing at Serie A clubs, namely at Roma and also at Inter. So Lukaku can be sure that he knows the atmosphere of football in Serie A very well. This will certainly make it easier for Lukaku to adapt quickly when playing with Napoli. Apart from that, as you said, Antonio Conte will also definitely make Lukaku the main player at Napoli. Therefore, I hope that Napoli's plan can be realized as soon as possible smoothly.