He's now 32 years old, and his performance may gradually decline. Therefore, relying solely on Lukaku will be risky for Napoli, who will be playing in multiple tournaments simultaneously. They've acquired a good alternative to him and can play him as a substitute. They won't be signing a good striker anymore, but I think they'll rely on the Belgian striker for another year. Next year, they could probably sign someone to replace Lukaku. Lukaku's performance this year will be decisive.
You say that as if at 33 the forward is no longer capable of playing as before. I think that Lukaku still has at least a few seasons left, but everything will depend on who will remain the coach of Napoli, if it is Conte, then Lukaku will play all this time, if they change the coach, then most likely everything can change for him too. Lukaku has a very good relationship with Conte, and Lukaku plays better under his leadership, apparently he is a player to whom you need to find an approach.