>>A prompt can be a masterpiece.

I don't think that word means what you think it means.

You have an extremely low bar for calling something a masterpiece.

A prompt can be clever, insightful, unique, and even uniquely productive.

But it is nowhere near the level of decades-deep skill and creative inspiration required to create art anything worthy of the label "masterpiece".

>>AI can be used as another kind of brush

Perhaps that is a valid analogy, but we do not give copyrights to brushes, no matter how much cost or effort was required to make the brush. The brush is not the only tool required to make the art. To continue the analogy, the artist must also select and mount the canvas, mix and color each shade of paint, build up the base layers, and on and on and on...

It doesn't matter if your "brush" is a five hundred billion dollar machine and you spend six months whispering to it to find just the right incantation to generate your file of pixels — SCOTUS is right, you have not make art to which you can claim a copyright.

And the starving student artist in their garage mixing their paints and using the dollar-store brush did make art worthy of a copyright claim.