From an economist’s perspective, “a topic” is not the good …

From an economist’s perspective, “a topic” is not the good being produced. The book is the good.
A topic is raw possibility. “Roman law,” “blockchain scaling,” “World War II intelligence,” or “how to build a chair” is not a book. It has no structure, no argument, no sequence, no research synthesis, no prose, no editing, no citations, no examples, no style, no marketable form, and no consumable intellectual product. The author transforms an open-ended subject into a finished expression. That transformation is labour, capital, time, judgment, and risk.

The economic problem is that books have high fixed costs and low marginal reproduction costs. Writing may take months or years. Research, editing, design, marketing, printing, and distribution all impose cost before revenue arrives. But once the work exists, copying it is cheap. In digital form, the marginal cost approaches zero. That creates the classic appropriability problem: the creator bears the fixed cost, while copyists can appropriate the finished product without bearing that cost.

So the question is not “why would anyone write about a topic?” Some people will, for reputation, patronage, ideology, signalling, teaching, or complementary income. The real question is whether enough people will invest in expensive, polished, risky, high-quality works when rivals can immediately copy the output and compete away the return.

Economically, without some mechanism of exclusion, the expected return falls:
Expected return = expected revenue − creation cost − opportunity cost − risk.
If copying destroys expected revenue while creation cost remains with the author, rational production declines. Not to zero. That is the cheap counterargument. But below the efficient level. Fewer books. Less investment. Less editing. Less research. Less quality. More short-form signalling. More derivative sludge. More dependence on institutions, patrons, platforms, advertisers, universities, or billionaires.

That is the part the anti-IP argument usually evades. It says, “The author still has the book.” Economically, that is almost irrelevant. The author still has the file, but the market opportunity has been diluted or destroyed. The loss is not physical possession. The loss is appropriability.
A chair is scarce at the point of use. A book is scarce at the point of creation. The printed copy is rivalrous; the expression is not consumed by reading. But the production of expression is still costly. Treating low-cost copying as proof that no protectable economic interest exists confuses reproduction cost with production cost.
The topic is not the investment. The written book is the investment. Without protection, the first mover pays to create the market, and the copier harvests it. That is not efficiency. It is a free-rider problem dressed up as property theory.
This is what they all paste over.