There is some discussion about whether nipple thrush exists or not - we should always remain open to new knowledge and not get fixed ideas IIRC, it was Dr Tom Hale who is sceptical about thrush, and well, he may be a man, but it's crazy to discount what he says just because of that....many leading bf researchers and medics are, indeed, of the masculine persuasion
The solid research on nipple and breast thrush is actually quite thin, especially with regard to thrush in the ducts. Many people do improve after treatment, but the body gets better by itself after many illnesses - just been hearing on the radio that is one of the bugbears with antibiotics, which people take when they don't need them, get better, and give the antibiotics the credit!
Personally, I used to think thrush was common. Now, I think if it exists it is prob less common than I thought - some women's pain improves with careful amendments to their positioning, and yet they have had people telling them 'it must be thrush'.
I don't believe that breastfeeding is supposed to hurt, though!