If you were a single issue voter on that particular issue, he was the obvious choice, but I don't think that issue is something on most people's minds or responsible for a significant proportion of single issue voters that could have been swayed to Trump.
Certainly, I think the rabid anti-natalism among the Democratic party is probably more responsible for a shift of any of said voters concerned about demographics than any declared natalist position that Trump advanced. He really hasn't pushed policy any further in that way than would have been considered very normal 10 or 15 years ago. With the sole exception of some states being allowed to implement abortion bans (although the extent to which that is really a natalist policy I think is up for debate, I don't think the majority of pro life voters conceptualize it in that way or expect it to have a measurable demographic effect).
Immigration and inflation and the cultural out of touchness of the opposing candidate all contributed to his win more than explicit fear about declining birth rates. That definitely contributes somewhat to the above fears, but it can't account for them entirely.
I feel qualified to speak on this being from a conservative family and being much more willing to consider Trump than I think the majority of the HackerNews audience is. Also, I'm located in Texas, where my point of view is perhaps somewhere around the median on a left - right axis. I've never once heard birth rates brought up by family or friends as a reason somebody wanted to vote for Trump. That being said, it would make sense if they did, and it's something I'd be interested in, on account of being a little more wonky than most of my family and friends, and thinking it's a serious issue that is still being vastly under discussed.
