Osimen has left Napoli for Galatasaray but it remains a regret that Napoli would have performed better if they had kept the star. As you said Napoli have a quality team this season and with this team they can definitely do well in the Italian league.
Football is not math and it does not work like math. Napoli can play well or bad with Osimhen, and we can not be sure about that. Because he left Napoli and they are playing well now, we believe they can play better if they have him to play together with Lukaku. But we ignore a fact that Osimhen and Lukaku are both traditional striker and they can conflict with each other if playing in a same squad.
Two strikers will struggle to support each other while a striker and a forward can play together more easily and comfortably.
At the moment we see this team's position in a very good position in the points table. If they continue to perform like this and gradually improve their performances, we can see them in the first place in the Italian league title race this season. However, even if Inter Milan is not in a good position yet, this team will be in a good position very soon and this team could be a cause of concern for Napoli.
They are playing well now and if they can maintain it till the half of season, they can actually join Scudetto competition with other clubs. Their performances are good now but they have a long season to complete, and many things can change in next 7 months. Napoli looks to be good enough to be in top four of Serie A when this season ends.