A postman's wage never supported wife, children and a mortgage as a rule.
Fair enough, I wasn't around back then to be able to speak from experience. But my in laws seemed to manage it, as did most of their peers from what they tell me. Mil did do some odd cleaning jobs, but she was mostly a SAHM until her dc were at secondary school.
Either way, it's irrelevant to how things are now nowadays.
Consider if every SAHP got a job and claimed childcare costs to do it - many of them paying very little or no tax at all - do you think help with child care would still be sustainable? Of course not.
We wouldn't be talking about every SAHP, because many SAHPs are funded by their partners wage, which is fair enough IMO. They might not be contributing financially, but they aren't taking money from the government to fund their lifestyle choice either so that's up to them.
But if every SAHP that claims child tax credits to enable them not to work got a job, then the money they are already getting could be diverted to funding their childcare. Then we would have more child are workers as well as parents being economically active, which has got to be better than paying for people to do nothing except for themselves.