This is a huge offer from Galatasaray and Napoli needs to accept this because they wont get an offer anytime soon or bigger then this one so they need to accept this. Osimhen wont play for Napoli and currently Galatasaray is the only real option in their case. So i think Napoli will fall under presure and accept this
If European teams doesn't want to meet his wage demand then this is the best option for him. He will continue to shine there, and probably their squad would be more strengthened to do well in European competition which would still improve his rating.
European clubs are seeing that he is at a disadvantage as a Napoli player after his relationship with the club fell, and now they want to try to get the best they can from him but he is resistant and fighting to get a better contract. Galatasaray loves him and he can continue to shine there till maybe opportunity arises to Europe again.