Awen,
Like SPB wrote below, diet and rest have nothing to do with amount or quality of breastmilk you produce.
As an extreme example, I spent 7 days in hospital where I could not eat at all, just saline drip and I was bright yellow on the verge of liver failure and stressed out of my head at being seperated from my 4 month old baby, but I still managed to express breast milk for my DD.
Your friend could of breastfed on mincepies quite happily if she wished to.
Your assumption that everyone's problems with breastfeeding are fixable is also inaccurate. As Aitch shows, some mothers can not nurse even with support/drugs etc.
Though I get your basic premise that with better support more mothers could continue to breastfeed. After all the stats show that most mothers wanted to breastfed longer than they did, so good support would help in this area. Have you seen the new goverment sponsered telephone number for breastfeeding support ?
You asked what would happen in the past if mothers couldn't breastfed and as for babies in the past, they would either die, be wet nursed or fed a basic formula homemade of boiled cows milk and sugar.