Which is Orwell's point, really. Abortion is death. It also is defensible. But because "killing" is hard and difficult to defend, we come up with euphemistic terms so we can stomach it.
The question is whether the justification suffices. For abortion, the justification is: "Because the mother doesn't want the child, we allow her to kill it in the womb."
If there's brain activity and a beating heart, there's really no scientific basis for saying an unborn human isn't a human being.
This goes back to Orwell's essay. We use euphemisms to soften brutal realities. A war monger may say we're simply bombing "savages", and an abortionist may say we're simply culling a "clump of cells."
Slaves were always completely viable. Bringing them up in a discussion about abortion is a strawman argument.
If you use viability as a guideline, you do bring into the debate people who were viable, but who through disease or accident are no longer viable. Should they be 'abort-able'? I think if the rules around abortion were flexible enough to apply to these people as well, they'd probably be solid enough to satisfy almost everyone's moral and legal sensibilities.
I don't think the line is drawn on the viability spectrum. A baby isn't viable until a month before it comes out of the womb, and that's a stretch. So I don't think this is a matter of people disagreeing with what constitutes viable because there really can't be any debate about that. I would argue that "potentiality" is a better term here.
The arguments, at their core, revolve around the fact that you have two living things who's interests may not be aligned. It's a question of who's interests will be favored.
Now, you can feel what you want about this, I don't want to get into a policy argument, but let's not dance around the issue, abortion is about the conflict between two abhorrent options.