I had this conversation recently with my sister, whose first child was adopted and whose second was a miracle biological baby. First baby arrived with three days' notice, and was ff'd. My sister was contemplating weaning her biological three month old, who had been exclusively breastfed up to that point.
She brought out the, "Well, X was formula-fed, and she's *fine*! And the kids I teach at school, you can't tell the ff'd and bf'd kids apart" argument (I think she was understanding the bf/ff argument to come down 100% on IQ).
It was a very difficult conversation as I didn't want to offend her, but wanted to encourage her to continue to bf her dd as long as possible (very difficult with her teaching job, she did wean at about 4 months in the end).
What I told her is, it's not just about your baby - it's about YOU. The longer you breastfeed, especially beyond a year, then the lower your risk of breast and ovarian cancers. It'll also lower your baby's risk, but yours is even more significant.
And I told her, lots of the things that bf reduces the risk for (like diabetes) doesn't show up until decades from now. She gave her first child the best food available to her at the time. And I hope that her daughter stays smart and healthy and "fine." But -- she has something even better available for #2.
I am afraid I rattled off all the "increased risk of" litany in bf vs. ff at some point in the conversation which may not have been helpful. I was so startled to even be having the conversation and for her to be so nonchalant about it (she'd been really sad when her friend weaned her child a month earlier).
I don't know, it is a difficult conversation to have. I think I wouldn't have really said anything (or maybe as pp's have suggested, "I'm glad your child is healthy!") -- if it had been a mere acquaintance. But I felt like I had to try to give my sister better information (she didn't even know that the recommendation is six months before starting solids; she waited that long with #1, but with #2 was planning to start solids at 4 months).
I do think a lot of it comes down to a misunderstanding of the science. And some comes down to defensiveness. But I think most importantly, the "my child is FINE!" argument works very effectively to end the conversation. It's a difficult point to argue, isn't it? I could easily have told my sister that her oldest child's persistent colds (nonstop really) are possibly less a result of daycare, and more a result of her immune system being less supported due to having to be ff'd. But that would have really inflamed things [even though I do believe it to be true - actually her dd had a dairy intolerance/allergy which I think she hasn't yet outgrown, since she varies from constipation to diarrhea now]. But if reality for the particular mother is, "Kids get sick, EIs are normal, my kid is happy and healthy in spite of them," then they're not going to recognize that for some families, EIs are not normal, etc.