You make a good point. People cannot seem to grasp that the notion of the the wine being juice was unheard of before the temperance movement began in the 19th century.
The other thing they seem to overlook is the fact that it was IMPOSSIBLE to make fruit juice prior to the development of pasteurization in the late 19th century. Without pasteurization fruit juice ferments on its own and this process begins almost immediately (especially with grape juice).
As far as the argument that Jesus would have made juice rather than wine because of purity, that is laughable. Juice must be pasteurized AND refrigerated if it is not drank immediately, the people in the time of Christ didn't have the ability to do either of these things and wouldn't for another eighteen centuries. Why would our Lord make something that would begin to spoil within minutes? Wine on the other hand DOES NOT SPOIL; it will turn to vinegar and may taste horrible, but it doesn't spoil.
They are just attempting to create God in their image.
I wonder, with what you said, how did the ‘juice’ last from the grape harvest that took place in the fall until the Passover feast, when 4 glasses of wine are drunk as part of the Seder? (could it be, fermentation? hmmmmmm..)