We don't disagree that it's a viable life. We disagree that all viable lives are worth preserving. I also don't quite understand how something that requires not-yet-invented medical technology in order to survive can "survive on its own." Surely, all you mean by "viable" is "can survive without invading another person's bodily autonomy." But that's beside the point.
They's be able to "survive on their own" in the same way that that people without working hearts can survive by having an artificial one implanted. Their ability to do that with new technology is irrelevant to the lack of survivability of patients without access to that technology.
Your argument is that the mother's autonomy is of primary importance, but after her autonomy is secure, the continued development of the zygote/embryo/blastocyst/whatever is next in line. I can agree with the first part, because I can see how violating the mother's autonomy can lead to tangible, experience-able negative outcomes. But for the life of me (pun intended), I cannot fathom your thinking in the second part. Your position appears to be: since the blastocyst would have developed into a baby if it stayed in the womb, and we have the technology to allow it to continue developing outside the womb, we should do everything we can to make sure it develops into a baby. You're leaving out a bunch of premises in there. Just because we CAN do something doesn't mean we SHOULD do it, obviously. When people decide what to do in a given situation, there is usually some consideration of what the results will be, and a comparison to what they would have been otherwise. So let's break it down.
The woman removes the blastocyst from her body, and we have a choice between (a) putting it into a machine that will provide the necessary and sufficient nutrients to turn the blastocyst into a baby, or (b) letting the blastocyst die. The consequence of letting it die is that it cannot develop into a baby. Your argument is that we ought to avoid this consequence, but you have not explained why.
Yes, I'm saying that we SHOULD do it. If we have the ability to save a life, then we should use that ability. If an otherwise healthy fetus is removed from its mother at 30 weeks, there's a good chance that it will die or have serious medical issues. If we place that same fetus in an incubator, there's no real difference between it and a fetus which stays in the mother until full term. This means that anytime an otherwise healthy fetus is born prematurely at 30 weeks, we should feel a moral obligation to put it into an incubator. At the point that it is removed from the mother, it is a patient with a treatable condition and should be treated just like any other patient with a treatable condition.
This is entirely independent of considerations about the mother. Because the fetus is viable on its own, the doctors should have a duty to treat it. If two pregnant women have a heart condition and their healthy 30 week old fetuses need to be removed and one wants to get a C-Section and have the baby placed in an incubator and the other wants an abortion, then the wishes of the one asking for an abortion should be ignored. The doctors can remove the fetus and have it develop into a healthy baby using the technology available to them, so it would be morally wrong for them to grant the mother's request for an abortion instead of placing the fetus into an incubator and allow it to continue developing.