With no *actual* formal knowledge, I think its decline & the way it's seen in society is directly linked to the major commercialism & convenience-driven attitudes that came about post-war.
Its decline arrived around the same time that the UK & the US saw popular culture drawn towards fast foods, TV dinners & less "make do & mend". My mum breastfed me back in the 70's when most others were opting for the convenience of bottles & formula- she was the only person she knew doing it.
This is really interesting - I wonder if there is also a regional bias to it - back in the 1970s and 1980s, our family lived in a much more rural area, where commercialisation was potentially slower to kick in! My mum and all my dad's sisters breastfed till a year, and never had the criticism that I've experienced today. My mum is so surprised when she hears I had comments from people asking when I'm stopping or remarking on why I was feeding so often and not in a routine, especially when LO was so young. She said no one would have made these comments back when we were younger - they trusted that you as the mother made the call about your baby's feeding. She used to sit and cluster feed me all evening, and also feed regularly in church and can't recall any hostility. My aunt bfeed until her son was 3 - the only negative comments she had were from my gran and it was only because my aunt had become so thin and wasn't eating enough herself, so my gran was concerned for her daughter's welfare.
That's not to say everyone breastfed - it wasn't for my mum's sisters. However, it seemed to be much more the norm to feed past 6 months. It was quite refreshing recently to go back to our roots and have my aunt say 'ooh did you want someone more quiet to feed?' as opposed to, 'oh, you're
still feeding your 9 month old baby?!' She just assumed that I was still doing it.
This is all opposed to my MIL who lived in the city, who breastfed in a very rational, structured and quantifiable way - e.g. x mins on each side every 4 hours, with gradual weaning off from 4-6 months, replacing by solid meals.
Of course there are other factors, and my observations are based on a handful of people, I just found these differences very interesting and wondered if there was any regional explanation to it!
Excuse my rambling - sounds like a really interesting topic you have there though. Hope it all goes well x
PS - forgot to say kellymom.com might have some helpful references