I read Jaynes for the first time about 30 years ago, and I had a fairly similar response to yours, but perhaps more positive.

However, interestingly enough, his work is still well-regarded amongst serious academics.

The brilliant, witty, and these days very eminent Daniel Dennett wrote a critical analysis of Jaynes where he points out that it's a just-so story and thus is not provable - but interestingly, he then goes on to say that Jaynes is very highly regarded in the field anyway and his theories are very consistent with modern cognitive science, while still being unprovable.

Dennett's most recent and probably magnum opus "From Bach To Bacteria And Back" lays out the state of modern cognitive science and Dennett's synthesis of these ideas, and it's a really good read too, even for people like me who aren't experts in the field.

---

Not from Dennett, but there are two counters to the very reasonable "Wouldn't isolated peoples still have bicameral minds" objection.

The first and best one one is that people in isolated societies are radically different from the Western scientific mindset, so how do we know they aren't bicameral?

The extra skills that Jaynes claims derive from a breakdown of bicamerality involve long-term or large scale thinking or planning involving modeling what other people will do. But these aren't skills that can really be demonstrated in a hunter-gatherer society.

The other counter is that if bicamerality were really a handicap and could spontaneously break down in any individual, who could then pass that idea to their children, is that these non-bicameral people would simply outcompete the others and be selected for.

I find that second one less convincing because the time scales are too short - thousands of years, not hundreds of thousands.