If these images were painted by hand, they would also be derivative.

But technologists insist that since one step in the chain of creation was algorithmic, the technologist gets all the credit and the original artist gets none.

I don't buy that there's anything special about "technologists" here. For example if I were to check the credits of the original Pirates of the Carribean movie, where is the credit to Keith Richards?

That's what Depp is doing right? Captain Jack Sparrow? It's a Keith Richards impersonation, but although Richards gets a part in one of the later sequels partly off the back of that, I don't see a credit for it in the first movie.

This is just how culture works. The pollination shades over from straight plagiarism, through homage and satire, until it's just unconscious assumptions and then it slips out of view entirely and even the creator has no idea it was there. Long before you disappear into the unconscious you're into a space where what you thought you were saying isn't what your audience understood at all. There are Warhammer 40K fans who don't realise that the Ultramarines are a joke. Yes they're blue, why is everybody laughing?

We have a court system which adjudicates whether works are considered derivative under copyright law. If there are recognizable subsections of the originals in the generated images, good luck with that court case — and good luck convincing the court of public opinion that you aren't taking credit for somebody else's work.