I agree that the most basic AI output could eventually come to surpass even the best human output. After all, the AI can draw upon ALL human writing of all time.

But to say that human altered text would be inferior is to say that the work is so perfect that any alteration would be detractive. On this point, I'm not so sure. Especially since what is "best" is often subjective.

Human alteration could vary the style or feel of the AI's work. Like how we would say Van Gogh and Leonardo Da Vinci are both great artists, but their differences are not on who is better, it's just different styles.