The "viability" debate, or the re-branded "when does this ball of cells become a person" debate, has been monopolized by philosophers and religious people. And as a result many women will die due to botched abortions https://en.wikipedia.org/wiki/Gerri_Santoro or the inability to get one in the case of complications (which are fairly common) https://en.wikipedia.org/wiki/Death_of_Savita_Halappanavar .
We can absolutely arrive, with a reasonable degree of certainty, when the ball of cells is capable of executing the processes we associate with consciousness. We can track the development of the neocortex, and monitor its activity to derive an answer to the consciousness question. We can do so via careful observation, and comparison with equivalent brain damaged patients, including those that we otherwise pull the plug on.
For what is the moral difference (from a consequentialist, utilitarian standpoint) between taking previously conscious humans with minimal brain function off of life support and an abortion of a non-sentient and non-conscious fetus? There is none, except for the fact that, in the case of a uterus, a human being is forced to act as life support.
Science can answer these questions. And do so with greater precision and accuracy over time. We can draw a map and arrive at a more enlightened conclusion. But the act of doing so, of so clearly putting a line down in the sand, makes most scientists uncomfortable. So they don't. And now many people will die because we created the vacuum that irrational belief filled.
We stopped engaging. They never stopped fighting.