This is surprisingly close to a selectionist account of behaviour. The only thing I'd add is that the contents of the first box, are themselves products of history.

"Priors", "weights", and "the real reasons" might feel/sound like explanations, but from a behavioural perspective they are things that also require explanation. Why these priors rather than others? Why these weights? Why this model? The answer is usually some combination of evolutionary, learning, and cultural selection (multi level selection theory).

So I agree that we only get to poke the system and observe behaviour. Where I'd differ slightly is that I don't see the hidden variables as the final cause. I see them as accumulated traces of past selection pressures, that way the box isn't mysterious so much as historical.