I remember waiting in the GP's surgery at my parents when I was seriously worried about DS. The receptionist let me and another woman sit in an empty surgery to feed (not that we wouldn't have in the waiting room, she just thought we'd appreciate the elbow room - and we did). I remember listening to the other woman's baby, who was about 3 weeks younger than mine (mine was 8 weeks at the time, hers 5 weeks) gulping down breast milk - as in, you could hear him swallowing, tell there was liquid going in. Then she changed a nappy - and suddenly the scales fell from my eyes. The HV had told me DS was doing fine if there were dirty nappies - but dirty nappies didn't mean "a skidmark", they meant "full of runny (as in typical BF baby) poo".
I look back at photos of DS at that age and think "how could I not have noticed how ill he looked?" He had sallow skin, sunken, dark-rimmed eyes and a sweaty sheen. He had also got so exhausted he could no longer cry at night: I had to set the alarm to make sure he fed in the night (another poster's comment upthread about how she supposed her DS would eventually have stopped crying sent chills down my spine - I think DS was very close to that point).
My parents' GP (a lovely woman from a farming family originally) sorted me out and effectively "gave me permission" to stop, saying I'd given it a really good go for 8 weeks and it was time to admit it wasn't working. She joked with me (admittedly this could have misfired with another woman, but for me it was the most useful thing that anyone could have said) that if I'd been a dairy cow, they'd have sent me off for meat by now. (Yeah, I realise that sounds awful out of context - but in context, delivered with a twinkle in her eye, just the sort of black humour I needed). But since then it's often struck me that I bet we know more about milk yield, and more importantly, variability in milk yield, in cattle than we do in human beings. Cattle - it matters economically, so study it in great detail. Women - who the fuck cares, just guilt trip them and make them feel bad about themselves.
But inability to feed really does happen. I've seen the incidence estimated at 1 to 3%. Which sounds tiny, until you think that this means in a typical primary school with say, 210 pupils on the register, without formula, anywhere up to 10 of them might not have made it through infancy without formula. As someone said upthread, mother nature doesn't know best. If you think seriously about evolutionary biology, it's survival at a species level, not at an individual level. Individuals are expendable, so long as a reasonable proportion make it. All this crap about "women are designed to feed babies" absolutely gives me the rage - there is no designer, and evolutionary forces don't care!