The article cites the relevant consideration: Sustainability. I thought about this the other day when I saw you reference global population projections in that thread on the new coal tech (to which I meant to respond, though it is a bit late now.) I am not endorsing some sort of extremist Zero Population Growth approach (for one thing, current projections show global human populations leveling off about midway through this century,) but finite resources put an upper bound on how high human population can rise before quality of life begins to fall. We can nudge that a bit with new energy sources and improved efficiency, but the Second Law of Thermodynamics is ultimately inescapable, hence it lies at the root of virtually not only all intra- but interspecies conflicts.

If extraterrestrial colonies become practical in the near future that might change (though only if those colonies rapidly become self-sufficient; otherwise they will only exacerbate rather than resolve the fundamental problem.) Until/unless that happens though anything and everything that permanently increases the human population, or any animal population, without proportionately increasing our energy supply must reducte overall living standards. Soylent Green was not just about plankton following animals into extinction; it was every bit as much about the food and energy demands of a New York City with twice its current population. It is no coincidence sustainabilitys widespread relevance emerged at the same time the longevity of postwar wealth and medicine became the Western norm.

Fortunately or not, I doubt we need fear continuing longevity increases overpopulating the planet to the point of social collapse. The increase in life expectancy since the Industrial Revolution looks more dramatic than it is because it continued so long, but there is real reason to question whether that pace is itself sustainable. It is considered an axiom that few Bronze Age humans lived past 30, but it is just as in/valid to say few Medieval ones lived past 50, and that few pre-atomic ones lived past 60. That is probably a net positive, but we may be at the point of marginal returns, because, having eliminated most deaths from wild animals, exposure, deprivation, injury and illness, Western deaths are increasingly due to the the human body simply grinding to a halt from wear.

Carefully and consistently maintained diet and exercise could well push the longevity envelope higher, but a world where most people live past 90 or 100 is not very plausible. As our bodies degrade from wear, making activity more limited and less rewarding, there is a real question whether anyone would WANT to live longer than that. Regardless, we are unlikely to find a way to replace/regenerate telomeres or reverse cellular free radical damage. It is hard to believe our life expectancy could nearly double, as during the 10,000 years between the Agricultural and Industrial Revolutions, or even rise ~25% as in the succeeding 200 years. An increase of a "mere" 15% over the next 50 years, comparable to that over the last 50, is plausible, but, in my unprofessional opinion, anything more is dubious.

Perhaps a better question, one directly bearing on sustainability but that its fiercest advocates resolutely avoid, is what longer healthier lives combined with the Second Law of Thermodynamics mean for the welfare state. Western social security and retirement medical insurance set retirement age at the current life expectancy, ~65, but retirement age has remained flat ever since, despite Western life expectancy increasing a full decade. Paired with falling birth rates, that is an actuarial disaster.

Maintaining retirement insurance when only 10-20% survive to use it and birth rates exceed replacement level is much easier than when 50% of people live long enough to earn benefits and population growth becomes static. Public retiree benefits were established for people who beat the odds, because surviving beyond life expectancy was unlikely enough retaining the abilty to work AS WELL was quite rare. Ironically though, few enjoying todays greater life expectancies are old enough to remember the basis of public retirement insurance. Most of us take retiring at 65 for granted (even when retirement benefits are inadequate for that,) but the programs were created in the days before bypass surgeries and artificial hips, when most were UNABLE to work past 65, if they even lived that long.

Sooner or later we must adjust to the new numerical reality, and rising life expectancies and overall health bring that day ever closer.