Napoli can get 100 million euros from Osimhen's sale. But the problem is some teams think Osimhen's price is a bit expensive considering his performance.
The price is very expensive and injury history of Osimhen is a big risk for any rich club that wants to get Osimhen. They can be ready to pay the cost but injury record of the player can make these clubs concern a lot.
On the other hand, Conte has worked with Lukaku and it seems that the tactics that Conte will use with Napoli will suit Lukaku's playing style. But Lukaku is old and will only be able to last for one or two seasons.
Lukaku is a safe replacement for Osimhen because Conte already worked successfully with Lukaku in Inter Milan. The striker still played well for AS Roma last season in Serie A and it's small risk for Conte and Napoli in using Lukaku for one or two seasons. For longer plan, ageing will be a biggest risk that can affect Lukaku contribution for Napoli but there is another risk, will Conte stay with Napoli longer than 2 seasons. He does not have good career history of long working time for previous clubs.
I think why Conte doesn't try to include Osimhen in his strategy. He is still young and still has the potential to be better in the next few seasons.
He knew that Osimhen no longer wants to stay and play for Napoli so it's reasonable to build up a plan or some plans without Osimhen for a new season.