A couple of reasons why authors might be worried about the press:
It’s a massive change to the technology of what they produce. This comes with lots of uncertainty and fear.
It commodifies books and massively decreases the unit price. Depending on how much you think quantity demanded will change, it could easily decrease your income. E.g, if the press came around and no one read any more books, it would be scary for authors and many would be out of work since now a single author can produce 100x more books.
Yeah, there can definitely still be imbalances/extra costs imposed on defenders but the point I’m making is that the projections people make are very often large over-estimates of what those costs will be.