The thing is, being a SAHP is an unnecessary luxury borne of privelege in the developed world and it's disingenuous to suggest otherwise. People have worked alongside raising their chldren since we lived in caves, and the idea of having no other job than taking care of your own family is little more than a century old.
Now please don't think this applies only to working as a SAHP. There are plenty of other jobs in the developed world which are unnecessary. Anyone working in the beauty industry, the hospitality industry, personal trainers, politicians etc etc. They contribute to the overall economy in our current affluent society, but they're jobs that will be first up against the wall come the zombie uprising.
The difference is that there is no obvious extrinsic financial worth in being a SAHP, which exposes them to more criticism and judgement than other "luxury" jobs, though in reality, they may be contributing more than, say, a nail technician, in terms of the increased earning potential of their spouse who is freed from domestic contraints.
And of course I recognise that caring for, nurturing and educating children is hugely important. But so is providing food, such as growing the crops we eat and the cows the provide meat and milk. In the days of yore, everyone had their own fields and cows and took care of their families that way. The needs of society have changed, so now while agrigulture is as important as ever, we recognise that it's not practical for us all to have a cow and a cornfield in our back gardens. If you do happend to have a hobby farm, it's recognised as exactly that - a luxury and a hobby. Fine if you have the resources, but purely for your own enjoyment/benefit.
It's the same with being a long-term SAHP - fine if you have the resources, but don't pretend it's anything other than a luxury that you personally choose to indulge in.