People don't have fewer children because the government tells them the world is getting overpopulated. They do so because they have economic freedom and they value their own happiness (however they define it). The only difference between now and before is that now people are free (have choices). BTW, who do you consider to be "undesirables?"
You hit the nail on the head. My wife and me are putting off having kids for a few more years because we are not ready for such a drastic lifestyle change. That probably means we'll end up having only a couple of kids, rather than 3-5.
The problem is, medical science hasn't figured out (yet) how to routinely extend fertility into a woman's 40's.