I think Napoli and Conte are not interested in Osimhen because Osimhen's wages are too high. Napoli management wants to cut their costs. This is why they will not keep Osimhen and Lukaku in the same squad. Moreover, Conte has wanted Lukaku in his squad since he took over at Napoli. Because Lukaku has played in Conte's squad in the past. And Lukaku was the best striker in Conte's squad. This is why Conte is so keen on Lukaku.
Conte is well aware of Lukaku's style of play. Lukaku is also familiar with Conte's match tactics. It is for this reason that Lukaku is likely to adapt quickly to Napoli's squad. Napoli are likely to see Lukaku perform well this season.
The real issue is that if he didn't really cut ties with the team, he could have been a marvellous striker for them, he could have been awesome, and there is no denying that he would be great for long term. However, we need to remember that he had damaged his relationship with the team so much that he was basically left out of the squad, if he stayed with the team then he wouldn't even be playing.
Even with Lukaku on the team, if Osimhen was good with the team and the manager he would have still played, but because the damage was beyond repair, we saw him sent on loan. They are not paying his salary and he will be let go whenever an offer is made. Plus they made a new contract, basically a longer one, but cheaper release clause, so it should be fine for him to leave now.