Neil wouldn't have been a good boss. yes he was a well respected street guy BUT he was constantly in trouble in the law.In fact he had just gotten out of prison at the time carlo died. Like it or nor Big Paul was probably the right choice because he stayed low under the radar and he knew white collar crimes which were the future of la cosa nostra at the time. I'm not saying Paul was a great boss but I definetly understand why carlo chose him.