> you are positing that an extended amortal life is at the expense of someone elses' birth.
> But why should it be?
That's the default, considering we live on a finite planet with finite resources, and even without immortality on the expensive of others is already quite the thing. I'd say the burden is on showing how that could change, not to mention why it absolutely must and will change... rather than just assuming it.
> we're talking SF
I'm "just" talking immortality, that I take for granted in this context, not additional things. At the least, those additional things can't just go one way, just because assuming immortality is assuming something hitherto impossible, doesn't warrant assuming other impossible things, while dismissing other possibilities, both possible and hitherto impossible, just because they'd spoil the parade.
Why would some live at the expense of others, or prevent others being born? Well, having "eternity" to lose, one must not let anything unpredictable happen, one certainly must not let anyone have the ability to harm oneself. The universe is too big to let anyone just get away and potentially hatch unpredictability. That is certainly a way to look at it, and it only takes a few with enough power to have that outlook for my dystopia to occur -- while your utopia would require nobody with power going that route, everybody always agreeing on "playing nice".
We can't even agree on a way so people don't starve and die for lack of water, we can't exactly agree to not ruin the planet, potentially leading to catastrophic shifts in rather short timespans, we already live at the expense of those who might be born after us -- but we're going to handle immortality well, if only we had it? We're greedy and murderous about shitty trivialities, about trinkets -- but we'll play nice when it comes to something like living forever? Seems unlikely, certainly not a given.
The possiblity of immortality combined with the fact of ongoing concentration of wealth and power, plus automation, might lead to a rapid depopulation of the planet indeed, but not by moving anyone anywhere. Why keep people around that are nothing but a potential threat, that serve no use, that are not even an exploitable resource, because they take up more space and resources than means of production requiring no workers that are orders of magnitude more powerful, and after some point plain unnecessary either way? After people "have everything", not by being content and loving life and the world, but by owning it personally, what they still need is for nothing else to be able to rise up.
(I also meant to write a short story once, about a little girl who skipped the weekly dose of the government mandated antidote for the biological weapon terrorists supposedly unleashed, say, 150 years ago, because she wonders if it's even true that not taking it is lethal, since nobody she knows ever failed to take it even once. She manages to hide the pain and the skin discolorations that appear after a few days, finally staying up all night the last night gritting her teeth before the day the next weekly dose is given out, sneaking into the bathroom early to wash off her sweat, before her mother wakes her. It was supposed to begin with her vowing to never do that again, then starting to ask questions about the past, but I never got beyond that.)