We aren't really Bayesian reasoning machines at all, and it isn't really accurate to speak of us having a prior. We choose a prior in order to use Bayesian reasoning to analyze a situation, and we seek to bend our natural reasoning to a Bayesian template in order to improve its accuracy, but we cannot wholly succeed in doing so. So the problem you raise should worry someone building AGI, but it's not realistic to imagine a human agent becoming so Bayesian that they swallow the Solomonoff prior whole and are literally unable to contemplate super-Turing Universes.

I don't think it's unreasonable, therefore, to adopt the Solomonoff prior as a useful model to aid reasoning and discussion, and rely on our human ability to make and adopt a new, super-Turing model if some more general prior would have favoured it.