I'm not the original poster, but here's an academic paper that found that 8:1 luminance contrast is optimal for reading, after which readability actually decreases: https://www.sciencedirect.com/science/article/abs/pii/S01698...

As far as I can tell based on what I've read, there can be a conflict between users with accessibility needs, where you may want as high contrast as possible, and typical users, where excess contrast may impair readability in some cases. The WCAG minimums are within the realm of reasonable for both sets of users though, so there isn't really an excuse not to meet those minimums.