You have mentioned an important point. Lukaku is a talented player. But he cannot maintain the consistency of performance. In the last 2/3 seasons we have seen Lukaku perform well in few matches, but most of the matches he is unable to score or assist. Napoli, on the other hand, needed a striker who could score regularly. I doubt if Lukaku can meet Napoli's needs.
Because he is older and older with time and his body naturally get worse, hence his performances get worse. In two latest season for Roma and Inter Milan in Serie A, he scored 13 and 10 goals that is not too bad for an old striker like Lukaku. In addition, when a player is old enough, he will struggle to maintain stability in performances and if we look back before era of Messi and Ronaldo, it was hard to find players who could score more than 20 goals each seasons. The goal productive era of Messi and Ronaldo with 40, 50 goals in a single season, makes us considering performances of other scorers are very bad, actually acceptable.
However, Lukaku spent the best part of his career under Antonio Conte. He was able to score a lot of goals with Inter Milan. And this team was coached by Conte. This is why Conte has wanted Lukaku in his squad ever since he arrived at Napoli. If CONte can use Lukaku properly then maybe we will see a better performance from Lukaku.
In seasons with Inter Milan and Conte, in Serie A seasons 2019 - 2020 and 2020 - 2021, Lukaku scored 23 and 24 goals for them. These stats would be very impressive but if we compare them with 40 or 50 goals from Messi, and Ronaldo, Lukaku is nothing.
https://www.transfermarkt.com/romelu-lukaku/leistungsdatendetails/spieler/96341