It's a really bad analogy. And the "cost" of working part time for someone who doesn't want or need to work is literally every single hour they spend working. If they're working 20 hours per week, that's 20 hours per week spent doing something they don't want or need to do. It's a huge cost.

and the cost IF you become a single mom at 40, and you’re unemployable? How would we quantify that cost?

We can agree to disagree. :) Hope you at least appreciated my different perspective.