I think he will leave this summer. But even if that happens I don't think Napoli will suffer a lot, they will most likely find another goal machine even if he's expensive.
They will earn around 50-60m from the deal, they can easily afford to sign any striker from any league - Benteke, Higuain, Lewandowski, Gomez, Aubameyang, Lisandro Lopez, Bony Wilfried, all will be available at the summer transfer market