More women in the workforce means more workers, more money in the economy, more viability for families, more progress, more development, more real demand for goods and services etc.
The problem isn't that there isn't enough income it the distribution of the income.
A single income family that is outbid for access to good schools through real estate prices may send a spouse to work to get access. The single income family that is now excluded may do the same. Eventually you reach a tipping point where every family is expending maximum income to fit into districts with good schools.
One thing I believe the book missed is that having two income families also has the impact of degrading the very same school districts as the book ignores the contributions of the non-working parent. Parents who are dedicated full time to raising children volunteer at school, have more well adjusted children and their social networks stabilize neighborhoods. The same forces that move families to send a spouse to work are the same that degrade the very schools that they want access to.
The result I believe is a game of survivor where couples with children bid for smaller and smaller islands of the middle class and more couples opt out of having children.
How schools are funded is obviously an issue in the US but also explicit tax transfers from single income families to dual income families through things like childcare credits and mortgage deductions also contribute to the demise of the single income family.
Senator Warren is obviously an advocate for women having the opportunity to work and have a career while the book analyzes the system wide implications of those individual choices. She makes a convincing case that the results have been for parents to assume more financial risk to raise children and that the risk is higher for single income families and higher still for single income/single parent families.
Some points that really crystallized for me from her book her analysis of household spending for a single income household in the 1970's compared to a more recent dual income household. Once you take out the higher mortgage payment (to get access to good schools), childcare (to support the second income), medical care and transportation (to support the second income) the household in the 1970's had more disposable income at the end of the month than the two income modern family even though the modern family has a much higher household income.
She primarily advocates for tax and policy changes to reduce the system wide impacts on families with children.
You aren't really making an effort to demonstrate this, or your earlier point that the "biggest issue" is making it "easier for women to work and succeed".
In Japan right now, it's not just that women are underemployed -- men are underemployed, as well. "Death by overwork" affects women as well as men. This would seem to speak to an overwhelming level of power on the part of employers, not a need for more workers.
I am really puzzled by the idea that the core problem in Japan has anything to do with the relative involvement of women in the work force.