I think the family would have remained in great shape, if not even better than it was. If it can survive a coup, having a guy like Genovese in power, then a 3 man panel while Genovese was in prison, I'd say it would cope pretty well with a smart guy like Costello remaining the boss.

I'd say there's a possibility Chin could have become boss. He was obviously smart and a good leader. I wonder who Costello would have picked as a successor if he had remained in power?