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It's gonna be quite a lot of additional surplus, and, they already do this. Surplus wine grapes get sold to concentrate makers, however, it pays less than it costs to grow them because concentrate is basically a commodity sweetener competing with sugar, corn syrup, and apple concentrate. Bottled grape juice is a small and shrinking market, and in that market, most people expect Concord flavour (these aren't that), plus a lot of bottled juice is reconstituted from that same commodity concentrate anyway.

Yes! I don't drink alcohol but have gone wine tasting with friends and one vineyard had grape juice from some of their grapes and it was interesting and yummy.

An interesting variety of grape juices would be very cool.

NA grape soda made from wine varietals is a niche product you can find in some wine producing regions here in the US and usually fairly expensive for a soda. All of the ones I've tried are pretty tasty, tough!