What is original to begin with? What's original about Bob Dylan's Blowin in the Wind? Certainly not the form! It's a standard AB folk song. Certainly not the chords! The melody? Sure, but very bounded by Western music theory and containing a number of common American melodic tropes. The words and specific rhymes have all be used before in previous poems and light verse. He used a standard 6-string guitar with standard tuning with standard guitar chords that have been strummed in similar patterns on hundreds if not thousands of previous recordings.

Beyond the technical skill required to create something there's nothing left but just a series of choices about how to rearrange what culture has provided for you. A truly original work would be incomprehensible to an audience in a way that a truly original language would be incomprehensible to an audience. The originality, the agency of the artist, stems from the choices being made, regardless of if that tool simulates a drummer based on the placement of notes on a grid or if that tool simulates a photographer based on the input of some key words.

Now, you can certainly say that if everyone has access to Stable Diffusion that the value of its output is relatively diminished and that is of course true. The same thing happened to drum machines. No one is that impressed by a four-on-the-floor beat coming out of a laptop and they soon won't be impressed by simulated painters but this is on a different axis of examination than originality.