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.
Lukaku is only 31 years old. And he is not in good form for 3 years. If a 28-year-old footballer performs poorly, we cannot necessarily say that he is too old to play well. I would say 28-32 is a suitable age for any footballer. At this time a football is most experienced. As experience is more, their chances of playing well also increases. But Lukaku has not fared well in the last three seasons.
However, it would be great for both Lukaku and Napoli if Lukaku's performances improve again with the help of Coonte. Napoli need a regular scorer. And for that they will rely on Lukaku this season. If Lukaku doesn't play well, we won't see a consistent performance from Napoli either.