Do you actually think that it is acceptable for the government to dictate that you not have children? If you were a legislator, would you vote for a bill which restricts all people to only one child? If you were a newlywed, and the government told you that you couldn't have a kid now, because the death rate had slowed to much, would you accept that? On what grounds can a society make that decision? And what about a society that refuses to do so? Do other nations have a right to force them to adopt mandatory birth control?

What exactly are the flaws in the "selecting for people who can't be convinced" argument? As far as I can see, the only way it wouldn't work is if failing to use birth control reduces the total number of reproducing offspring. If that is the case, then nothing needs to be done about it, and people who refuse to use birth control will dwindle away. As to Social Darwinism, that's the idea that superior societies win out. The idea of selection in human populations is just natural selection.