40 year old have kids all the time without any medical intervention

46 year olds not so much and if they do deliver a baby it’s a donor egg one

It's not crazy to want another baby at 40, and women who can conceive a second or third without intervention after 40 are not unicorns. It has just become culturally atypical to do so because of the birth control and family planning options that have been available to us for the last several decades. Anecdotally, two of my great-grandmothers had healthy babies in their early and mid 40s. Of course there are no fertility guarantees at any age, but if you've already had a baby your chances are certainly higher.