What is your point?
At one time is was legal to own slaves, and kill them as the owner saw fit
At one time it was legal to kill mormans,
At one time is was legal to kill american indians,
At several times it was legal to kill jews,
Just because something is "legal" does not make it right.
One day our society will move past the "its legal" excuse, and start doing what is right. Killing innocent children is not right.
Maybe one day, which it might take 100 or 150 years for society to mature, we might be able to add "At one time it was legal to kill innocent unborn children" to the list of crimes against humanity.
No one advocates the killing of innocent children, so you're starting off on powerful rhetoric but shaky logical ground. The debate is and essentially always has been around what qualifies as a child. You might consider a 5 week old group of cells a baby, I do not. Is it life, possibly, but that's a different question. You also assume that life of whatever quality is always better than death, another point I'd dispute.
The reason abortion is legal is not because it's "right", but because there are complex competing interests at work and the overwhelming majority of citizens, legislators and judiciaries who examined the issue at this point in time, given the competing interests and the options available, has found abortion acceptable if still unsavory and unfortunate.
If in the future there is perfect contraception which eliminates any unwanted pregnancies, I believe that humanity will collectively let out a sigh of relief. Medieval doctors used to drill holes in people's heads for unrelated ailments and we find that barbaric now, but we don't really judge them for it given they didn't know what else to do. Abortion is regarded by most as a lesser evil, not as an endorsed and recommended method of birth control, but many of those who rail against abortion also oppose contraception. If our hypothetical future person wants to condemn us for acts of necessity given our limited technology, he would probably also be mystified at resistance to methods that would prevent the choice from having to be made. Oh, and the plastic bags found in the remains of so many women's chests. Did some plague destroy the lungs of women in the 21st century requiring silicon replacements?
If abortion were completely illegal now in all circumstances, futureperson might just as easily look back with horror and say "Wait, if a woman got raped the government would turn her into a baby factory against her will?" This scenario is more plausible than yours given that women will take over the earth soon.