Great discussion!
I believe it's not a 'personal, individual choice'. I always thought it was. Until I moved from a Northern European country to the UK. Our choices of a couple have changed, whereas most of my mummy-friends in my country have just continued with both partners working 4 days/week, like I wanted. I have now become a WAHM, doing 90% of the house/childcare work, whereas he is pursueing a career. Why have we changed? Different flexible working laws, different mat/pat leave policies, Different quality and cost of childcare. All external factors that society/politics can change.
Changing those factors would make it much easier for me to argue my DH should do a step back at work, find cover for school holidays and thus continue to work myself in a professional career. He might not easily accept it, but at least the arguments that 'it is impossible to work 4 days in this job' wouldn't work any longer. His reasons for not 'doing his share' of the house/childcare work seem to be supported by policies in the UK.
I just wanted to mention that most of my friends at home bring in quite a bit more jointly (in 2x4 days) than my husband does on his own (in a very good career). Besides it makes them less vulnerable in case of redundancy of one of them or in case of divorce.
Of course, you don't want to force this model on everyone, and that is a danger and I agree with Mills that even though I might prefer this, it doesn't mean most mums do.
At the same time I believe it is a waste to society to make it so difficult for well-educated, experienced women to stay into senior jobs when they have a child. Many choose to stay at home because they don't want a senior job and all the pressures it comes with AND look after a child. But I do think their choice would be different if there were more options to work flexibly in senior jobs.