I can't talk about every IA, but lots of IA models do not "scrambles it and rearranges unlicensed material". Neural networks for instance, try to imitate our brain, the same way you take inspiration in an existing game to create your own. And yes if an Art create by an IA is similar enough to another existing art, then that's plagiarism like how lots of humans do the same way.
And No, AI will never replace humans in making art, what will happen is the way we do Art will shift completely. For instance, instead of painting yourself you may now talk to an AI instead and artists in this "new era" will be most likely people that can take the most of the AI to create the best art pieces. It may be quite similar to what already happens nowadays with sculpting (we still have people carving on rocks) but it's way more common to find people sculpting 3D art in their PC (using Z Brush for instance) and 3D printing them or just uploading them.