Certainly none of the SAHM that I know are doing anything outside of paid work to contribute.
On the other hand, nearly every SAHP I know does some kind of voluntary work (breastfeeding support, sling libraries, Home Start, advocacy for children in care, helping in school or running the Cub pack etc). Several are SAHPs to also help take care of elderly relatives or older children with additional needs.
I think there are very different situations within the catch-all term of "SAHP".
I definitely agree with working towards some kind of financial security independent of a partner though. That said, there is a big difference between lots of women's experience of well-paid, career-type jobs and most working-class women giving up an admin or retail job to raise kids. There has often been less of a career ladder to step off, and those working parents would be left vulnerable anyway in the case of a break up. There would be BIG lifestyle changes for every family in the event of divorce.
£60k around here (East Mids) would be plenty to bring up 4 kids, especially as part of a long term plan with savings being made in the cheaper primary school years for university, driving lessons etc.