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 definitely a good player and he recently had good performance while playing for teams in Serie A like Roma and Inter Milan but he was usually unstable and he didn't stay in the teams he joined there. Even though there is some news about Lukaku returning to Chelsea and since they need to get the team improved he could be a good helping hand in Chelsea.
It seems Conte was smarter and since he saw Osimhen is not going to stay, before they lost Osimhen, he asked the team to hire Lukaku.