fourmummy, you make good points, but you have (sorry) misunderstood the evolutionary arguements. Evolution shows us that breastfeeding and breastmilk is robust as a 'system' across the human race. It does not show us that in every individual case it was always a breeze with no problems.
Breastfeeding as a 'system' is definitely not 'precarious' - your word. In a small no. of individual cases, it can be....not many of them, as too many individually precarious breastfeeding mother-and-baby-pairs would have made the 'system' fail.
In addition, as soon as you are talking about human beings living in groups, you make many biological functions into social ones, with norms, taboos, expectations, rituals whatever - this is especially the case with gendered biological functions (you can probably think of several examples yourself - puberty and menarche; sexual relations; birth; conception).
Breastfeeding is no different. It becomes hard to see where the medical knowledge (itself culturally-mediated) ends and social expectations begin, in cases like your own, where your baby lost weight. It may well have been that your baby could have overcome his initial (and yes, concerning) weight loss without readmission into hospital and treatment with formula. In a society without scales or expectations of weight loss/gain limits, and without formula, nothing would have happened at all. 'Nothing' would not necessarily have been a useful nutritional response - further days of weight loss and lack of sufficient milk might have led to worse consequences, who knows? Or perhaps another woman in your tribe would have fed him, and you would have been cossetted and a spell might have been cast by the local wise woman to make you make more milk :) Maybe the psychological support and confidence would have helped and maybe encouragement to feed more often day and night would have worked.....I don't know and nor can anyone.
You achieved a lot in maintaining mixed feeding to 6 mths. Most women don't. Mostly, early formula does derail bf, but for a mix of social, medical and nutritional reasons which are hard to separate.
Your anger and sadness have made you vehement about the 'message' of historical breastfeeding. It's not 'crap'. Breastfeeding was the norm, and if a mother died, it was still the norm - someone else stepped in to feed the baby, with their own breastmilk.
Our current generations are 'worse' at breastfeeding, but not for physiological reasons.
It is common in non-Western societies for babies not to be exclusively breastfed, and to be supplemented with animal milks or grains mixed with water, or honey...lots of things. The studies done on this show that usually this is not so much because breastfeeding is failing, but because of the impact of social, cultural and sometimes religious factors - pre-industrial, non-western, 'under developed' societies are no less subject to these influences than we are.
This is not a good thing for babies' health, and health workers in these areas of the world try to increase the exclusivity of breastfeeding, for this reason.
That was a long reply - and does not get to the heart of your feelings about this, which is sadness that your baby was not breastfed in the same way as his sibs. It's normal to be sad about this, because for you, breastfeeding is unique, special and part of your identity as his mother. BUT he was breastfed, and he had that closeness and contact with you, and that was still unique and special....and he is loved and valued. Your regrets about not fully bf him sound hard to cope with, and I wonder if some counselling or other talking therapy would help you? 'Cos being sad and angry is understandable but for it to remain as strong and as bitter as it does all these months on is not good...IMO.