The idea that grape juice was used instead of fermented wine is, historically, a relatively new one.
Grape juice was most certainly not used for a very simple reason, it would spoil and get moldy after a day or two. Even plain water, when stored in skins or chests will become undrinkable after a period of time.
Remember, they did not have refrigeration in apostolic times. To preserve food, certain measures had to be taken. For example, drying fruit and smoking or salting meat, were common.
Alcohol is a preservative. When grape juice is fermented into wine, that wine can be stored for a very long time. This is also how beer developed. As a means of preserving something to drink. (By the way, that is why many people who travel to countries with questionable water will often drink beer.)
I have heard it said, but I do not know if it is true, that wine in apostolic times, had a lower alcohol content that wine today (which is usually around 12%). If that is true, then the wine then would be less intoxicating.
If someone insists that they were drinking grape juice, then ask them if they would be willing to drink grape juice that has been stored in a container with a loose fitting lid outside during the summer for a week.
