Well, here's why I'm skeptical (or let's just say opposed if you prefer) - overpopulation is already a problem, and substantially lengthening life expectancy will only make that worse. Basically, I'm afraid we're closer to solving our medical problems than we are to solving our planet-supporting-our-population problems. If we solve the latter in tandem, then I'm on board.
Or are you only talking about quality-of-life improvements, rather than lengthening life expectancy? Assuming this is even possible, I think the problem with it would be the psychological toll of a death to the survivors - while always sad, the death of an elderly loved one is less jarring than the death of a loved one in their prime ("it was his time") - so keeping everyone in their prime right up until their death would eliminate a powerfully useful coping mechanism.
Yes, as a first step, I'm only talking about extending the high-quality-of-life period for everyone. And I know it is possible, because we're already at it and there is no magical barrier stopping us from doing this on a biological level. I also don't believe that "we shouldn't do it because it makes us less sad when dying people have wrinkles and white hair" is an ethically valid argument against this kind of research.
There are two viewpoints that make any further discussion pointless. One would be the belief in any sort of afterlife, and the other one is the conviction that human consciousness has at best a very limited intrinsic value and can hence be discarded without further consequence. If you don't hold one of these beliefs, I wager there is a high degree of probability that you basically agree with people like me that we have to do something and that things cannot should how they are now.
Of course, this kind of technology will over time lead to drastically longer lives as well. When it comes to aging, there is no way to do the quality thing without the quantity thing.
Overpopulation is not a problem in wealthy, industrial countries. These countries will always have advanced tech before everyone else. So, for example, if I were to live 200 years in Germany, producing maybe 0.8 children over the entire time, I'm not crowding up the countryside. Sure, I do require resources, as does everyone else, including some Bangladeshi guy who died at 35 because of some horrible tropical disease und who nevertheless multiplied by a factor of 20 over the same 200 years.
We need new and sustainable technologies to cope with both corner cases anyway. This idea that progress has to stop now is a horrible mistake. We left the realm of our fellow animal sisters and brothers to embark on a journey of knowledge and discovery. In the course of this endeavor, we became powerful individual minds capable of achieving pretty much anything. Well, we're halfway there now, we can't just stop suddenly, that would be the end of the race. We have to move all the way through to the other side.
Or are you only talking about quality-of-life improvements, rather than lengthening life expectancy? Assuming this is even possible, I think the problem with it would be the psychological toll of a death to the survivors - while always sad, the death of an elderly loved one is less jarring than the death of a loved one in their prime ("it was his time") - so keeping everyone in their prime right up until their death would eliminate a powerfully useful coping mechanism.