Lukaku might be can recognized as flop player in Chelsea because on that team Lukaku never able to playing consistent but in my opinion when loaned to Inter Milan and Roma Lukaku performance is quite impressive even in Roma he is top scorer of this team on Europa league so what i want to says is Lukaku is suitable to playing in Serie A and everytime he was playing at this competition although with different clubs but his performance is quite stable so might be this is the main factor why Conte want him instead of Osimhen
Lukaku has just make his debut with Napoli and he can scored and bringing Napoli to beat Parma and i think this is good start for him especially after Osimhen didn't including to Napoli squad of this season and loaned to Galatasaray then Napoli hasn't have sharp striker and probably Lukaku can solved the attacking lines of Napoli
We had expected it before, when Napoli have 2 players like Lukaku and Osimhen then they have to choose one of them, of course Lukaku is the best choice because his salary is lower and his performance is also impressive. We will see how far Lukaku can play with Napoli who have a good record under the same coach, as you said Lukaku has made a good start by scoring a goal in the last minute and that also proves that Napoli made the right decision. Now it's Osimhen's turn to prove Napoli made a mistake by wasting him.
In a comparison between Lukaku and Osimhen, I would definitely put Osimhen ahead considering recent performances. However, Osimhen's wages are higher than Lukaku's. However, we have seen better performances from Osimhen in the last two seasons. But Lukaku hasn't done so well in the last few seasons.
But since Conte likes Lukaku, keeping Lukaku in the squad is probably a good decision for the Napoli management. However, if Osimhen and Lukaku were at the same time in Napoli's attack, Napoli would have been stronger. But Conte is not interested in Osimhen. However, expect to see a better performance from Lukaku. But it is not certain whether he will succeed.
I also think the same as you, Conte's decision to sell Osimhen, is very unfortunate, whereas if you think about it, if Conte had paired Osimhen with Lukaku, Napoli would most likely have great ammunition for the front sector, even becoming a big threat to Serie A clubs and Napoli will be a club to watch out for in Serie A this season, but the decision that Conte has taken is a unanimous decision and it cannot be denied that he knows better about Napoli's future development.