First of all, the moral question is of paramount import. A system premised on immorality, regardless of any perceived positive externality, is a negative and immoral system and thus inferior to any moral system governing the same set of activities.
Ignoring that however to respond to the specific arguments in question -
Insofar as the legalization of abortion might make one more willing to have intercourse, and to exercise fewer precautions when doing so,
and insofar as the use of abortion enables a subsequent pregnancy at a much sooner time than had one seen pregnancy to its natural end of child birth, therefor enabling a rapid, subsequent abortion,
and insofar as a person who is morally reconciled with abortions might receive several,
and insofar as the birth and care of a child is a significant event requiring greater attention and therefor allotting less time and availability for intercourse, let alone another child birth, than that of a person who has aborted and is free of such responsibilities,
the potential for population growth is stunted in the presence of permitted abortion compared to that of prohibited abortion.
It seems all but certain that while there might have been some fifty-five million abortions, there would not have alternately been fifty-five million child births were abortion prohibited.
Furthermore, of those purported fifty-five million abortions, in a number of those cases the child could not possibly have been saved and abortion was used as a last resort to save the life of the mother.
Therefor it is a farce to beg the question, "What if those babies were "saved"?"
We can, however, assume that some number of those abortions, in an alternate reality which prohibited abortion, would have been birthed and thus subject to your consequential catechism, so I will address your other questions as well.
"Who would take care of them if the families couldn't?"
Michelle Bachmann, for example.
I am aware personally of three families with adopted children, and I myself, as well as my sister, were adopted into a loving family.
I understand that you want to avoid any moral quagmires but is your argument truly that those who cannot be properly cared for - whatever that standard may constitute - should instead be killed, and that this is the proper solution? I find that notion to be utterly repugnant, repulsive and disgusting.
I would personally rather live a poor life than none at all.
"Should crack addicts and unfit mothers be forced by the state to give birth?
Basically, in the long run, aren't we just creating more government dependency if we force mothers to have babies that they can't take care of?"
This question seems a ruse.
The fault in your line of reasoning is that you suppose that either "us," or the state, have any control whatsoever over whether or not a crack addict or unfit mother gives birth. That responsibility rests with the two (and in the case of rape, one) parties who willfully initiated the act of procreation.
In reality, the only agent which could properly be said to 'force mothers to have babes' is nature. The natural result of intercourse between two humans, wherein her egg is fertilized by his sperm, and that co-creation attaches to her womb and derives nutrients, is a human baby. It is not the body of the woman, as some might say. It is a distinct entity which is composed of both parents' makeup. To illuminate this certainty, imagine that during the course of tongue kissing your spouse, a number of your own cells come unattached from your tongue and remain inside the mouth of your spouse. Are these cells now officially the body of your spouse? To say such would be disingenuous and flatly false.
Do not suppose from my counter-arguments here that I am in favor of any sort of government solution or mandate regarding abortion. I am not. I prefer to approach the question from the philosophic rather than legal and political perspectives.