Lukaku has always been a great footballer, the problem he had was that he never fit in Chelsea system so he had to be loaned out constantly when he was there, finally he figured out a way to permanently make a move. He also got injured a while back, which made him a bit slower, but he is still talented, maybe he is not as fast as he used to be, but his speed was never the intended attribute that teams looked for him, his positioning, his heading, his shooting and technical abilities were all the things that made him who he is.
On the very first game, we saw how important those are, he helped his team make a comeback on the last minutes, and that's why I think it's important for Conte to get who he wanted, and he basically got rid of Osimhen.
Napoli's insistence on keeping Osimhen has cost them dearly. Unable to get the transfer fee they wanted, Napoli insisted on keeping Osimhen and this created tension between the player and the team. Do you think that if a player wants to leave a team and a good price is paid for him, it is not logical to let him go? I don't know if Napoli did it on purpose but now they are change to replace Osimhen with Lukaku. However, Lukaku is not in his old form and is not a player who can contribute to Napoli in the long term. Maybe this season he can fill Osimhen's void, but Lukaku has a stability problem. Napoli need to solve the problem with Osimhen because every team needs a player like him.
Lukaku is a great player, but he is not unleashed to the peak of his form in every system. Some managers know how to get the best out of him, some don't. He was never the one to take the ball in dribble past 5 to score a belter, but he is someone who creates a lot of space for his other attackers, takes runs in behind, and gets into dangerous positions more often than most others.
The fact that Osimhen remained unsold is a massive flaw on Napoli's side. Osimhen made it clear long ago that he wants to leave, and had talks ongoing throughout the transfer window. Napoli was adamant about getting a high price for him, and lastly couldn't even sell him. If they negotiated better, they could have agreed to a decent offer from one of Chelsea or PSG, but they were greedy. I think they might need to sell Osimhen later on for even lower than what they were offered this window.