Literally everything you said here except for that last sentence is baseless.
A) The vast majority of abortions were not of those with genetic "defects". In fact with no abortion we would be more likely to have far more producers than we would have net recipients, meaning that as a proportion those who are condemned by the nature of their birth to depend on others would be less of a net drain on the system. Furthermore, your conclusion does not follow your presupposition - in a system such as the one you describe our overburdened and straining social welfare system would have already collapsed, and the ability of the government to provide a huge new entitlement such as single payer would be roughly nil.
B) Women are born more than men; nature's way of allowing for death in childbirth. Except, thanks to modern medicine, women don't die in childbirth a anything resembling the 'natural' rate - far, far, from it, in fact. This means that as the number of births would be significantly higher (carrying with it a higher number of female infants) even as the number of birth-deaths plummeted, that our ratio of women to men would, in fact,
climb slightly. It is
abortion which produces fewer women to men, as so many engage in sex-selective abortion, especially in cultures that more heavily value sons over daughters. Feel free to do a search for "China's Bare Branches", and you'll see the same effects in an extreme case, but only an exaggeration of our disparity, not a difference in type.
C) The "Abortion reduces crime" shibboleth became popular after the Freakonomics guys came up with it, and was then utterly destroyed by someone who
actually studies the history of violence,
Steve Pinker. It's Pop-Social-Science.
D) Again, see (B), we would not be facing a surplus of young men - that is the issue that is faced by countries who perform
lots of abortions (such as China), not countries that do
not perform abortions. That being said, killing off surpluses of young men through warfare is a tactic used by states who feature no significant political feedback mechanism with punitive power (autocracies), not representative governments.