Bottom line, the risk of going after a witness after the damage has already been done outweighs the rewards. Where the mob is still willing to kill, in the case of rats, is if they suspect somebody is informing or about to flip.
Now I'm willing to bet there are a ton of wiseguys out there who disagree with you. Nothing has hurt this life more than informants. If they knew where a majority of these guys were, they would go after them. Whether they've testified or not. It would make the next guy think twice about cooperating. There has even been heavy discussion about going after the families of informants.
