I agree with the 'Wouldn't that be nice', moving the conversation on is the best tactic and that it reads like she's more frustrating with wider social situations than with you working specifically from what you've written.
Times have changed, as they always do. For most of humanity, families and communities worked together to deal with survival including caring for and teaching children how to be part of that. The recent changes are more individualisation of people and families rather than surviving being harder - most mothers have always worked, just usually with the family. I don't think this would change your MIL's mind much though to get into.
I would not throw a barb at your husband's earnings to try to dismiss your MIL's opinions as many are suggesting, unless you both agree to that beforehand. I really don't see how making a jab at your husband is meant to teach your MIL anything, especially when she specifically said that the government should be helping to enable this rather than that men should.
My husband and I took turns being at home with our kids. Only our oldest (now 20) went to childcare and it was a disaster - it did cause him harm in ways that took awhile to repair. I live in an area that was known to really struggle with having quality provision at the time, especially for little ones with additional needs as my oldest was, and it's not improved that much over the years so while I agree preschool provision can be great for kids, much like as many mention with SAHP & is often discussed about schools, any care for kids is only as great as the provision that's accessible to them and their families.
When preparing to have kids, I do think that is something to consider - what is available in your area and how you would manage if you have a child who cannot access childcare. There is a cultural image that, while horribly expensive, all the paid-for childcare options are there, and for some kids, families, and communities, they aren't.
women being the only mammals to go through menopause, which stops reproduction after a certain age but they continue to live for many active decades afterwards with the purpose of supporting their own offsprings to raise more children.
Humans aren't the only one, if we define menopause as cessation of female reproduction rather than menstruation as most mammals reabsorb rather than shed off the uterine lining. It's been well-recorded in some species of whales and there is growing evidence that many mammals including chimps and mice do so if they live long enough - it's just most species don't live that long enough in the wild to get to that point of running out of releasable eggs. Humans may do so earlier compared to our maximum lifespan compared to the average for mammals in part because that gave evolutionary benefits as the theory suggests, but we're not unique in having menopause in the safer domesticated settings we've designed.
I don't think this would shut up someone who wants the government to support SAHPs.