No they're not forced to sell, but they will never get offer like it again and could easily reinvest that in Italy. Not only that, he may very well want to move on - he's only on 3.5m a year. He's also been there for 5 seasons, to get over 10x that 5 years later is great business.
They won't replace him with a better defender and they'll prefer to be around the top 2 or 3 than selling him for 60-70m which is pretty low in the current market. As I said even Leicester don't want to sell Harry Maguire for such price and they have less ambitions than Napoli, so not sure why they'll care to sell him unless a +100m offer arrives.