I guess you could try just reading the decision. But the answer to your question is already there, you're just not seeing it apparently.
It's not:
Quote:
viability makes sense because that's when viability is
but rather:
Quote:
Viability makes sense because it allows for a way to resolve the tension between respecting womens' right to self-determination and a legitimate state interest in legal protections for the unborn.
It allows for the constitutionality of legal restrictions on abortion to hinge on whether or not those restrictions might be implemented without
necessarily requiring such a large and long-lasting burden on the mother, specifically.
It's also probably true that in practical terms the workability of the compromise depends on viability being at a late enough time, given current laws. But the principle works regardless, e.g. you can imagine (though it may not be realistic) some sci-fi scenario where viability was earlier. Following Roe, you could argue that the state
could restrict abortion earlier, but only if the state also provided women with the option of essentially giving up the child by inducing birth earlier and placing it under someone else's care, thus removing the unconstitutional burden. Viability marks the point where such a solution is minimally possible.
In the present, the practical consequences are more that viability is late enough that, if you haven't decided to abort by then (and I'm ignoring cases involving health risks here, and just talking about unwanted pregnancies), there's no real compelling reason to make that an option, because there's really no need for it. But the logic of the decision doesn't hinge on that, specifically. It hinges on the possibility of separating the state interest in preserving the life of the fetus from the necessity of the mother's involvement in keeping it alive.