Ronaldo can bring more confidence for Napoli players. Also, italian league a bit have a slow playing rythm, so if Ronaldo join to Napoli, he can give a chance to bring trophy since with his skill and his performance, still applicable for Serie-A. Don't forget about money, merchandise sale can help their finance. The only problem is his salary, i'm sure maybe hard for naples to pay him.
Ronaldo still talented player and I don't thing Napoli will get trouble if sign in Ronaldo in next window transfer open, actually have positive impact side if Napoli want to sign in Ronaldo due merchandise and Ronaldo's shirt will sold out fantastic depend on several team before with Real Madrid, Juventus and Manchester United become most sold Ronaldo shirt. Have slowly rythm I think Ronaldo doesn't need adapt with Serie A culture and become easy how to get in Splatti scheme games play.
But Napoli won't to sign in Ronaldo if his contract not break the contract with Manchester United in January, actually Ronaldo have several months left contract with Manchester United until June 2023 and seems not any one club interested to sign in him if not be free transfer player.