Value is something that can only exist in minds, and if everyone were to suddenly change their minds about that which is valuable, all the labor in the world used to produce a previously desired product wouldn't mean a thing.

This is mostly an argument about definitions. If everyone's minds were modified so that people would start valuing the eating of babies, there is still a clear sense in which it won't become a right thing. If you are talking about that which most people value at any given time, then certainly it depends on what most people value at that time, and people's minds are part of your definition that controls its meaning. If instead you form a fixed designator for whatever people currently value, it will still be pointing to the same thing if people in the future start valuing different things, and minds of future people won't be involved in this definition and won't control its meaning.