it contains no socially necessary labor and its value vanishes. So there must be a use value first, then labour theory of value is applied. Demand does not determine value itself. If technology cut production from 10 hours to 5, value will fall even if demand stays constant
Similar arguments like “something can take 100 hours to make and be useless if nobody wants it” against LTV are also stupid. For a commodity to have exchange value it must have a use value. If an item is produced that nobody wants…