You may say that it is unthinkable. But it's just you choosing one cost instead of another. Being a decent human or not is a choice, being a single paying alemony or a couple hating your kids is another choice. And there is a price to all of them. That's what a choice is.
A choice is not about doing whatever you want. That's omnipotence. A choice is chosing what to do with the piece of block the life tetris gave you.
But of course, none of this argument matters because I'm answering to HN readers, a niche a very educated people that know all about pregnancy, contraception, abortion and that having children has a huge impact. This post is, after all, about somebody interested in the process enough to trigger a whole debate it.
So my comment is to be taken in that context.
This attributes way more intentionality onto having children than actually is the case, in almost half of cases.
In 2011, 45% of all pregnancies in the US were unintended, and 18% of all pregnancies being unwanted [1]. That means that for the nearly 4 Million births in the US yearly (using 2011 numbers), the intentions you state about how to think about time management with the additional kid likely does not apply for about 1.8 Million of them.
Speaking as a father of three, your statement is discounting a number of things - including personality type - and I find that those with kids who do a lot are predisposed to doing so, how they manage that with kids varies by their predispositions.
So its naiive to equivocate having kids to taking on a new hobby or a job - for various social, legal and ethical reasons. They just aren't the same.
[1] https://www.guttmacher.org/fact-sheet/unintended-pregnancy-u...