By that time Costello had ingratiated himself with the upper class. He'd learned new ways to deal with enemies. The way of the wealthy and politically powerful. Take note of a) how long Genovese lasted on the street as boss, b) where he ended up, and c) how he ended up there. Costello stays retired to keep the peace with the Genovese faction, Carlo is now essentially the top guy with Costello continuing to profit in several business ventures while also being a defacto consiglieri to his old family....while Vito Genovese rots in federal prison.