I think it depends how much you want the grapes... There is no guarantee that the grapes are actually from the original cultivar vine, rather than just wild grapes or a hybrid or whatever. (Unless you saw the old vine and were like "whoah that's a gnarly old vine!" Unless you want grapes
enough to be okay with ending up having to plant a new vine after finding the existing grapes unpalatable, it might not be worth it. If you're really excited to salvage that arbor it's more likely to be worth your time. You could wait and see if any grapes survive to harvest, and taste them and see what you think, but usually the grapes here fall off or get eaten by birds before we can try any because we don't tend the vine.