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.
I don't think the idea of selling Victor Osimhen was only done by Antonio Conte, the club's management is also involved but I wonder what could make the player attract so much hate from the club he once helped to win the Serie A. Even if Osimhen and the club's management had a misunderstanding the best way was to dialogue and see how they can retain the player at the end of his contract in Napoli but it's unwise for Napoli to lose such an important striker without caring about the future of the club. Well, that is bygone now let's see how the new signing Romelu Lukaku will be able to turn things around for Napoli as Osimhen did for them. However, Lukaku cannot measure up with the standards of Osimhen in terms of energy and goal scoring so there is absolutely no comparison among the two players.