The justification for Bayesian inference, that the posterior will eventually converge to the true distribution, breaks down unless your prior has support for a

The justification for Bayesian inference, that the posterior will eventually converge to the true distribution, breaks down unless your prior has support for a good approximation to the data-generating model. So without a good model of how polling results map to actual voter distributions, the Economist model is guesswork, to a large extent.