When does a mother finish being a mother?
Well, in my case, never, unless my daughter dies before me, I guess.
But at what point does being a mother stop limiting or interfering with my potential to do other things? I'd say, probably when the last child is weaned. Because after that anyone can do the hands-on caring and parenting needed, and those responsibilities can be shared (and in our family, my husband does more of it than I do).
So: time from conception to weaning? Somewhat less than two years, in my experience. Let's say two years per child. Even accepting your argument that most women need to have two or three children, we're talking about, at most, six years or so of the woman's life where study, work, and social involvement might be significantly limited by being a mother.
Given that my life expectancy is sitting at about 82 years, that leaves me 76 years where I can do just about anything! Ok, take out the 18 years before adulthood and I'm still left with 58 years, where I am a functioning adult not limited by being a mother.
Now, tell me why, in those 58 or so years, I should sit on the sidelines of life, instead of finding whatever ways I can to contribute to the benefit of those around me?
OP, the only quibble I would have with your post is with the idea that confining women to childbearing and housework is an agrarian-society thing. I think it's more of an industrial-revolution thing, and that before that women often worked as hard alongside men in farming.