If you imagine the publisher’s business as one that divides most of the consumer’s dollar between two core stakeholders in the supply chain — the retailer and the author — you’d have a pretty accurate picture. The publishers, at least theoretically, decide what the retailer’s “working margin” will be with their discounts and agency agreements. And they decide what the author’s share of the proceeds will be by the advances and royalty rates they offer and agree on through their contracts.
These are the essential, and basically non-substitutable, trading partners for a publisher. They can choose a different printer or publicity firm without changing the character of their business or their economics. But the author relationships are existential and defining and the intermediaries who reach the public and enable the consumer transaction are indispensible.