Looking at Lukaku's age, who is currently 31 years old, this player is actually not that old. It's just that bringing in this player cannot be used as a player in the long term. Napoli will probably only be able to see the peak of their performance in the next 2 to 4 seasons. After that, Lukaku will be no longer young and perhaps his performance will decline. It's just that I still like this player's style of play for now. He showed his sharpness when playing for the Serie A team. Like when he was at Inter Milan and at Roma last season. So Conte's decision to bring in Lukaku can be understood. Because maybe he will only use this player for a few seasons. After that I'm sure Napoli will look for other young stars. But for now Napoli immediately needs an experienced striker to quickly increase the strength of their squad again. And bringing in Lukaku is one route they consider worth trying. However, it seems that Conte has another reason for bringing in Lukaku, namely because it seems like it will be easier for Lukaku and Conte to understand each other. Because they have worked together in the past.
Lukaku is just ap layer they know they are getting what they are buying for and they are not going to be shocked with whatever happens. That's sometimes good enough, yes it is not a marvellous trade or anything like that but it is at least a known trade and we should probably consider that this could be a good investment for them. I know that it is going to take a while for him to be getting back into match shape but he is still a good player, and good enough for Napoli level as well.
You need to remember that not every team tries to win the league and while Napoli win it in recent years and would love to win it again, they are aware that they are not going to get it again. In that case, Lukaku isn't a bad player for a team that is pushing for fourth place at the best case in their mind.