"Shot" is not a technical term and that's a pretty broad statement. The harness is made up of over a mile of wire and it's not likely all of it needs to be replaced. In my Automotive Electrical class, my students were required to determine the cause of a problem, then locate that cause. Replacing a whole harness or running a new wire were not acceptable responses. Sometimes a large section gets damaged such as from falling down onto hot exhaust parts but even that is easily repairable.
You didn't say what the symptoms or problems are or why they feel a whole new harness is needed. If they can't find the cause of a problem they might be hoping it's a bad connection or corroded splice, but the chance is greater more new problems will crop up by tearing a bunch of wires out and putting different ones in. Connector terminals can become stretched and make intermittent contact. Wires can break off terminals. Plastic wire loom hangers can get broken and overlooked allowing more problems to develop in the future.