I think that is commonly the reason why some businesses do offer discounts, but note the person above replied to "but for rules-following merchants taking cash isn't any cheaper than paying the credit card fees" which seems to have specifically anticipated exactly that.

Yes, exactly. The business team at my (rules-following) company were ecstatic when municipalities around us began disallowing cash payments during COVID. Taking cash is, for us, directly 2-3% (and actually more, if you count labor, which is difficult to track) more costly than the vig on credit cards.