You might be right about the self-fulfilling prophecy. But this effect might already be in effect, as people might already have a gut feeling, while nobody cares (or knows) about it.
A trivial instance of this would be release planning in software. After our initial delivery plan my team gives a confidence measure, one release it was quite low, which was an important signal since we could discuss why that was and what we could cut to increase the confidence. Without that signal, we'd have overcommitted, and suspecting as much but without common knowledge might have even delivered less.
You're right. But I think the single-value, hyper-transparent nature of prediction market can create an institutional feedback loop that's more powerful than what arises from the diverse, individually held opinions that others don't know about.