> I’m absolutely certain there are plenty of NFB members who will gladly provide free screen reader accessibility testing for apps they use if the developer is responsive to feedback.

You could be right, but keep in mind that this may not significantly lower the amount of time required for developers to understand and remediate problems. Users have wildly differing levels of technical experience, so you may end up with plenty of feedback that you then have to spend hours understanding, sorting, de-duplicating and following up on.

Not to mention the fact that, bluntly, users who aren't being paid as "experts" just may not be that willing to shit all over your product. I have encountered more than one case of a limited subset of screen reader users reporting a positive experience with a component which broke every rule in the book, and caused very real problems for users outside of that core group.