Not quite never, and the predictions of your various theories are also priors. So suppose I'm a physicist in the 19th century. And I've got two theories 'Classical Physics' and 'We're wrong about everything'.

This bears no resemblance to the actual history. How much resemblance was it intended to have? You say in another comment:

And I don't, by the way, put this forward as an account of 'how classical physics fell'.

But your reason for that is only:

Those guys were using classical logic.

There were several known problems with classical physics in the late 19th century, and "classical logic" vs. "new improved Bayesian logic" has nothing to do with how they were resolved.

1. The black body spectrum could not be explained.
2. The photoelectric effect (going a few years into the 20th century). It took a certain amount of energy to knock an electron off an atom, but light of arbitrarily low intensity could still do it. Only the wavelength mattered: there was a wavelength threshold but no intensity threshold.
3. EM theory predicted an absolute velocity of light, but Newtonian mechanics defines no preferred frame of reference, and the Michelson-Morley experiment failed to find one.

If you've got an immense prior belief in a theory that can explain anything at all, then yes, that's hard to shift.

Having a prior so immense that it's hard to shift is a problem anyway. But what is "immense", and what is "hard"? I pointed out here that ordinary people are quite capable of updating against 80dB of prior improbability (and if their posterior certainty is of the same order of magnitude then they've updated by around 160dB).