Well, I doubt we'll ever get around entropy and energy conservation but who can say? That upper limit, just by known techniques, is still pretty absurdly high, given billions of years with billions of solar system each with billions if not trillions or more people in each I wouldn't be surprised if someone found an escape clause. I mean a billion worlds with just Earth population and normal modern people would have more thought occur every minute then the whole human race has had in its entire existence. The sort of Malthusian limits at full solar let alone galactic or bigger are safely in the zone where you can ignore them under the assumption being far more like to be dealt with better, one way or another, down the road. Currently the Second Law is not really an issue against continued expansion, there are more immediate technical rather than fundamental phsyical hurdles to leap.

The ever-thinner sliced pie issue and Malthusian problems have usually not been good guides to what should or did happen thereafter. I'm not even gonna touch the Soylent Green thing.

Sustainability has always been an issue all the way back to hunter-gatherer times and it's just a modern conceit in the face of evidence and common sense to assume our ancestors weren't well aware of these issues. I doubt there were a lack of chieftains who looked around and knew they couldn't realistically expand their territory and knew they couldn't squeeze much more out, or that their people were unable to comprehend the issue when he came out and said "Okay guys, we can have less kids, be pickier about the ones we let live, kill of our sick and elderly, exile or kill troublemakers an undesirables, or try to invade our neighbors, or some combination of those... preference?" and feudal cultures regularly practiced deliberate population control.

Even if you can't find a way to just get people to not age the analogy might be a car you just keep replacing parts on, the last piece of the original will likely still be around long after you've had to replace some parts repeatedly and its still the same car after every piece has been replaced, after all most of your body is not composed of atoms that were in you when you reached adulthood, as opposed to a car or refrigerator which usually hits the junkyard long before most of its atoms have been replaced.

I feel obliged to point out that its very hard to stick a satelite into orbit but we sure have hung a lot of them up there since the notion was first seriously contemplated less than a human life ago, back when it took entire buildings to house computers with less power then an iPad. We haven't known much about DNA until relatively recently even compared to rocketry and computers.I think it is way too soon to be talking about limitations on what we can or can't do with DNA, especially since we've been able to start printing the stuff. I don't even think its all that absurd to think someone might have an 'oh, duh' moment in the next couple decades and come up with a cheap and easy never-age technique. It's only been about a decade since we really knew much more about DNA then its general function and existence.

It's like liquid crystals, those got discovered in the late 19th century about the same time they were noticing pus-filled bandages had what they came to realize was DNA but it wasn't really seriously studied in a mass effort till the 50s and 60s, but we went from 7-segment digital watches and clocks when we were kids, to the cheap and total replacement of vacuum tube 'Radiation King' TV's and monitors of our youth. When I was just starting formal study of physics the Buckyball was the new cool thing as was real, genuine superconductors, the C60 buckyball was supplanted by the carbon nanotube and very recently by graphene, a super material of uncalculable worth... which is manufactured with pencil lead and scotch tape and now with DVD players. I'm not trying to diminish Novoselov but the wonder material he developed is essentially an 'oh, duh' and I'll be astonished if mass production takes more than another decade to get rolling and if it isn't incorporated into virtually every damn thing within a decade thereafter.

Well, the solution will depend on the problem. Obviously if a longevity technique meant you were 80 before you were as physiologically beat up as the average 65 year old the solution is to raise retirement to 80 and tell people not to be stupid about it. If the technique means you hit the equivalent of 65 at 65 and chug along unable to work for another century then you have to flatly tell people that if they can't make their own arrangements too damn bad. If it is just 'don't age' then you tell people retirement is an outdated concept and they should seek 'sabbatical', work, save up cash, and go on vacation for a couple years then go back to work, or work less but constantly, as circumstance and preference dictate. There may or may not be constraints, until we see what those are it's a moot point. It's the equivalent of worrying about the effect of video games on children and internet porn on relationships when you've just manufactured the first silicon chip, nobody did that of course because none of them envisioned either of those. Until we have the problem literally in our lap the pros and cons can't be weighed with pragmatism in mind, only basic morality.

Let's say you had 3 miracle drugs and that people joined the workforce at 20, retired at 60, and died at 80, just to keep the math easy.

Drug 1: People reach maturity at 40, retire at 120, and die at 160 - no real meaningful change in resources, people still a quarter growing, half working, and a quarter dying, except that someone working in their field for 80 years is probably way more productive than one who has been for 40.

Drug 2: People reach maturity at 20, retire at 60, and die at 120 - this is not so good, they spend one third rather than one half of their life producing

Drug 3: Mature at 20, retire at 140, die at 160 - This is very good, people produce for 120 of their 160 years, 75% of it, education and retirement cost are minimal compared to production.

These all involved people living longer but are totally different problems and concerns, trying to develop a 'plan' for those events prior to knowing which one isn't doable because they have nothing in common in practical terms

I don't think most Americans in our age rang take retiring at 65 for granted, Joel, we're noted for being rather cynical and people have been talking about the inevitable collapse or rollback in age on SS for a long while.

The question on the table though isn't social security but whether or not longer lifespans are immoral or undesirable. That has to be an 'in and of itself' discussion same as something like capital punishment. If you say 'we should execute all felons because they are eating valuable food' you're in a different argument and one that doesn't revolve around life expectancy, that's just another factor in a classic argument, that whole Calculus of Despair, which we always have to do, even if currently the nastier form is having to pick between the the drunkard, the seventy year old, and the mother of three hit by a car as to who gets the single liver we have for transplant. Those are separate from morality of longer life in of itself.