I hear of people having this conversation with mothers of nursing 3 month olds, let alone mothers of nursing three year olds.
I've recently worked out a potentially wonderful response which I am calling the Mona Lisa response.
When people say challenging things about breastfeeding, I think it is worth quietly continuing to breastfeed, and breathe calmly, and just feel the tension. Because the person challenging you is the one with issues, and they are trying to tranfer their anxieties to you. So take time just to feel their tension, not to act on it, but being aware and conscious and fully present in the moment. It's not your tension, it's just crackling around in the atmosphere of the room. You don't have to engage with it in any way.
And when they have finished their rant, smile, like the Mona Lisa, and say "ah well, horses for courses, it's what suits us for now" in the tone of voice which signals clearly that the conversation is now over.