I think you're mixing up some terminology here.
Article 100 of the NEC defines grounding as, "Establishing a connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to some conducting body that serves in place of the earth." Grounding provides a path for conducting electrical energy to earth to prevent arcing, heating, or explosion during a lightning strike.
The NEC defines bonding as, "The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed." As it relates to a communications system, the primary purpose of bonding is to equalize ground potential of, and eliminate static discharge between, equipment. If a potential difference exists between two objects connected by a conductor, electrons will flow along the conductor from the negatively charged object to the positively charged object, damaging electronics in its path. The flow of electrons happens until the two charged objects are equalized and the potential difference no longer exists.
Other than that, I can't really be sure what you are asking.
You say, "if an existing galvanized system were grounded to the cold water main where it enters the house couldn't the old galvanized conceivably remain grounded via a jumper?" For one, I think you mean bonded. Second, I don't know how the "old galvanized" and the "cold water main" differ.
The NEC requires the cold water main, as you call it, to be bonded within 5' of the entrance into the building. If it's not metallic, no bonding. After that point if it transitions back to metallic, I don't think that isolated portion needs to be bonded.