The validity of eyewitness testimony is vastly overstated and having it all be treated as if those giving it were lying would be an improvement because they are often not telling the truth because their memories are imperfect, though they do believe themselves to be telling the truth.
>It would be legal to publish child pornography.
Those who produce it would still be hunted down with extreme prejudice and any potential market would be immediately priced out by legally produced computer generated images. If anything, it would better allow us to hunt down the producers, increasing the number of children rescued. It would also stop the courts from ruining the lives of children who sext. Not perfect, but I wouldn't be so quick to assume the costs are greater than the benefits.