Agree mousema and moonunitalpha.
I have quite a few friends who say they didn't have enough milk. But when I ask them how they knew, or what was happening to make them think that, they describe what I now know to be pretty much the normal behaviour of the breastfeeding baby.
There was no unusual weightloss, or failure to thrive - they were just 1st time mums who didn't understand that their baby could feed frequently, or constantly! and have growth spurts, and wake so often in the night for feeding...They were expecting (as was I, ha ha ha) a baby who fed once every couple of hours, had in a nice solid feed, was full and went to sleep quietly between feeds.
So in the 1st few days, they found BF-ing hard, totally time consuming, and they worried their babies' behaviour meant they were too hungry. So they fed formula, their babies seemed contented, went to sleep for ages, and so they concluded that their own breastmilk wasn't enough. It may well be that after that, their supply was affected - all of them stopped BF-ing totally quite quickly, none of them carried on mix feeding.
However, I don't think all of them wanted to carry on BF-ing, as well, where is where it gets complicated and very sensitive. Why do I think that? Because they've said more privately how relieved they were to stop, and they say they find FF-ing easier (greater freedom, shared night feeds etc). I don't think that they are lying about lack of milk - I just think that they found BF-ing very hard, and a shock, and the anxiety about 'how much milk are they getting??' (which I had!) made formula seem not just attractive but necessary.
I find it really hard to listen to. It seems crushingly insensitive to say 'sounds like you may have had enough milk, but that you got anxious because you were overwhelmed and didn't have enough support.' Because it's done, now, isn't it? And sometimes I've thought - 'you really didn't want to BF, you've just told me how you know someone whose sister is a consultant in Harley Street, who says there's no benefit to BF-ing after 1 week, how you hated being the only one able to feed your DC....' I don't know, but it's not as straightforward then as 'I really wanted to BF but couldn't.'
Obviously, they may have not had enough milk, but it was never a professional diagnosis for the mums I'm talking about, it was their own judgement. Which is where another sensitive problem comes in. 'You know when your own baby's hungry, don't you?'
Well, not necessarily. Why on earth should a 1st time mum with no practical experience of BF-ing know the difference between a frantically cluster feeding 10 day old, and one who is genuinely being underfed? I wouldn't know, and I'm an intelligent parent who tries to do my best. Parenting is about a learned craft as much as instinct, and there was so much I didn't know about normal BF-ing until I found MN and Kellymom.
Anyway. The problem arises I think when you get a lot of women citing not enough milk as a reason for stopping - so it seems like it's actually a very widespread and common problem. Which then sows seeds of doubt in other new mums minds. As a 1st time mum, I was SO worried in the early days about whether DS was getting enough milk or not. I had him weighed whenever I could, I forensically studied his nappies, even used a torch to try and see his fontanelle through all his hair....! I still think all babies should come with a 'tank gauge' attached.
So - I don't share the OP's anger or attitude, but I do think it's unhelpful for the idea that a LOT of women suffer from dangerously low milk supply to become widespread.