I've talked about forgery in art before. My thinking has evolved, a little, from there.
In the entry previous to this one I suggest that a wet plate photograph would lose value, were it revealed to be a digital photograph edited to look like a wet plate photograph. People would likely be angry and upset. There are really two things going on here. The first is that someone lied to us, and we are justly irritated by this. The second is that we are forced to adjust our thinking about the photograph, reducing its value, and this is something we're not terribly happy with. Quite apart from the lie, we are forced into a situation where we must admit that we were wrong, in some sense. The image isn't as good as we thought it was.
This is somewhat analogous to what happens when a painting is shown to be a forgery. It's not a Vermeer, it's a van Meegeren. It's less valuable. Again, people are angry because they were lied to. Again, they are angry because they are forced to devalue the work, they were wrong about the painting.
In the case of the painting, however (and this is where my thinking has evolved and changed) there is an actual devaluation. It is perhaps minor, but it is real. When the painting was a Vermeer, it fit into the body of Vermeer's work. It taught us a little about Vermeer's evolution and his ideas about painting. It helped to fill out a portrait of Vermeer for us, as a sort of anthropological artifact of the man's life. When the painting is revealed as a forgery, it takes all that back. We have to unwind our understanding of Vermeer a little. The painting's value as a source of information about someone interesting drops to zero (well, not quite, van Meegeren was interesting as well). The painting's connection to other work is broken, it is no longer a valuable piece of a puzzle, it's just an oddly shaped piece of cardboard after all.
Is this devaluation proportional to the outrage? Probably not. Still, there is a genuine loss of value here.
A fake wet plate photograph shown to be a fake loses no such connection. To be sure, it's no longer connected with wet plate photography, but so what? There is no corresponding network effect for wet plate photography. We do not need more wet plate images to truly teach us about wet plate. We lose nothing of our understanding of wet plate when a fake is revealed. We have thousands, perhaps 100s of thousands, of these things already, and anyways who cares about a portrait of wet plate? In this case we're mad simply because we were lied to, and because we have our nose rudely shoved in the fact that we decided the photograph was good because it was wet plate, not because it was actually good.