It is all well and good to say we are allowed to kill to stop killing, but that is not the only reason you cause deaths. People have probably died today because I had slightly larger lunch. I don't feel guilty about this. But it is in many ways true. The extra food or its proxy in money could clearly have saved lives, if deployed better. I don't consider that murder.
One does not need to question whether or not the fetus is alive in a sense equal to oneself as @RexKerr does, or to question whether or not this is a 'justifiable murder' as @Melody might, because it is not a murder, it is a withdrawal of support from a dependent, to preserve your own dignity.
While this may not be optimally moral as a motive, it is at some level acceptable, and should remain legal within reasonable bounds. I don't have to feed the homeless people on Lower Wacker who appear to be on the verge of starving to death. It is not that I can't. We are going an average of 15 mph sometimes. I could pull over, get out and give them food without bugging the guy behind me. I could afford it. And the only real reason I don't is because it is below me -- it would be strange, and abasing to do it.
And it is not about responsibility. Even if I were somehow responsible for their fate, say they were my ex-employees, and I had fired them because of my own incompetence, it just would not be required. If I don't have to do this for homeless people, why would I do it for a random potential child I don't know (yet)?
(Then again, if I feed someone regularly things become different. You do have to support people who become members of your household, legal fatherhood is not biological, but based on the reasonable expectation of support. And if I foster a dependency, and then withdraw it, I am more at fault than if I cut it off as soon as I am aware an expectation is developing. So as arbitrary as the one we have is, it is not illogical to move it back as detection becomes more certain and stigma decreases.)
The common reference on this is Judith Jarvis Thompson's 'violinist example'. Hers is much better storytelling, but this is the gist:
If you wake up sewn to someone else, as a consequence of doing something criminal in the past, are you obligated to remain attached to this person, if the attached person would not survive the surgery necessary to separate you (until seven or more months from now)? Even if this could well destroy important aspects of your life? Is your right to freedom and autonomy totally overruled by the newly dependent person's life?
If we assume life is inviolate -- a much stronger position than that abortion is questionable -- then your answer is 'yes'. We should do everything that is not as bad as actually killing before allowing someone to be killed. You claim that 'This case where life is inviolate' ... 'is already well established in law and medicine'.
But then why is anyone (below a given age) dying of kidney failure? We could choose random people found doing things they should probably avoid doing, test them for tissue matches, and sew them to any matching candidates on the kidney waiting list so that they shared a blood supply. We could just do this to everyone in that position, and they would survive. If life is inviolate then we should because that is what inviolate means, that all other considerations are secondary. Clearly the result is nonsense. No one would just ignore vigilante rescues of ailing children by attaching them to randomly selected adults. It would be legal to reverse these procedures. But we are OK saddling random teenagers with being attached to dependent children for months at a time, because it is not done by vigilantes, but by Nature.
Edit -- warding off purposive obtuseness:
The being-sewn-to-a-kidney-failure case is not parallel to the transplant case. In that case someone is going to die anyway. I am arguing that it is still OK to let people die for other reasons than to save a different life. So this is an argument where the fetus is life, it is not necessary for anyone to die, but someone dies anyway, because our obligation to help one another is limited.
The OP's main point (as I see it) is that life's importance can only be measured against other lives, so there is no room for any other consideration here. But that is not how things work anywhere else. As just demonstrated, the assumption that life can only be compared in value to another life leads to utterly absurd requirements, like that all medical procedures that might save a life at the cost of someone else's freedom are still obligatory.
Things that lead to absurdity are usually considered to be incorrect. So the OP analysis is wrong. In particular, no one should accept the premise that life can only be compared to other lives. If our obligation to protect life is never comparable to choice, they you would be obligated to save anyone dying of kidney failure who can find you. It is in your power, so you would not be allowed to choose to do otherwise.
And the issue there is autonomy: the right to choose one's future; and nothing else. The question is where that right kicks in, not whether it exists, or whether 'life' is involved.
But the opposite position, in which autonomy, at least at the level of compelling someone to use their body for your purposes, is equal with considerations of life and death, is equally questionable. We have wars, and we have throughout history often drafted soldiers. We still register all men for a potential future draft, so we have not decided this is outright immoral in any obvious way.
That kind of implies the society can legitimately take pretty complete control of your body for short periods, with a good reason. (More control than a pregnancy does. The pregnant woman, after all, could still live where she was used to, and control her time as she had before, within reason. And for longer periods, the randomly obligatory draft stint has never been shorter than 18 months, AFAICT.)
We have done this totally randomly, over long periods, and when we were not actively at war. So there does not have to be an imminent threat, or a specific cause, other than a standing obligation to the system that provides your safety. And the impact does not have to be fair, especially not between the sexes, since we ever only draft men.
So, obviously neither "preserving life" nor "choice about the use of one's body" are basic principles, since they cannot just apply to children and women, respectively.
To my mind, abortion needs to be legal not based on some basic moral principle, but because we, as a society do not take care of mothers or children, because our societies are capitalist in principle and on purpose (even the nominally socialist ones).
every mother were treated respectfully, rather than some being stigmatized morally for 'illegitimacy', and others being dismissed as non-competitors because they enter the 'mommy track'; and
she could bring the child to term and hand it over into a system where it could always reasonably expect a life fully comparable to that of other children, on average; and
she were compensated for her lost time and pain (not necessarily fairly, but as soldiers have been -- on a scale that is minimal, but not insulting.); and
if her other rights were not already selectively systematically degraded relative to those of men long-term by previous generations taking unfair advantage of women's willing participation in the process of pregnancy (or like in the case of men and war or crime, we acknowledge we are biased, but institutions attempt to balance this.)
then we would not need to keep abortion legal. But I don't foresee that happening. It can come close sometimes in historically wealthy, rather stable, extremely homogeneous, under-reproducing societies. But throughout most of the world this is just intractable.