Gaining more years of healthy life through progress in medicine is a change with no downside, to my eyes. That many people strive to find problems, and that many more people seem disinterested in gaining more years, disinterested in eliminating age-related suffering and frailty, is a mystery to me:
The greatest benefit of life extension is the continued existence of the individual who remains alive. Each individual - apart from the worst criminals - has incalculable moral value and is a universe of ideas, experiences, emotions, and memories. When a person dies, that entire universe is extinguished, and, to the person who dies, everything is lost and not even a memory remains. It is as if the individual never existed at all. This is the greatest possible loss and should be averted if at all possible. The rest of us, of course, also lose the possible benefits and opportunities of interacting with that individual.
People would be able to accomplish far more with longer lifespans. They could pursue multiple careers and multi-year personal projects and could reliably accumulate enough resources to sustainably enjoy life. They could develop their intellectual, physical, and relational capabilities to the fullest. Furthermore, they would exhibit longer-term orientations, since they could expect to remain to live with the consequences of decisions many decades and centuries from now. I expect that a world of longer-lived individuals would involve far less pollution, corruption, fraud, hierarchical oppression, destruction of other species, and short-term exploitation of other humans. Prudence, foresight, and pursuit of respectful, symbiotic interactions would prevail. People would tend to live in more reflective, measured, and temperate ways instead of seeking to haphazardly cram enjoyment and activity into the tiny slivers of life they have now. At the same time, they would also be more open to experimentation with new projects and ideas, since they would have more time to devote to such exploratory behaviors.
Major savings to health-care systems, both private and governmental, would result if the largest expenses - which occur in the last years of life today, in the attempt to fight a losing battle against the diseases of old age - are replaced by periodic and relatively inexpensive rejuvenation and maintenance treatments to forestall the advent of biological senescence altogether. Health care could truly become about the pursuit of sustainable good health instead of a last-ditch effort against the onslaught of diseases that accompanies old age today. Furthermore, the strain on public pensions would be alleviated as advanced age would cease to be a barrier to work.
I do not see true drawbacks to life extension. Certainly, the world and all human societies would change significantly, and there would be some upheaval as old business models and ways of living are replaced by new ones. However, this has happened with every major technological advance in history, and in the end the benefits far outweigh any transitional costs. For the people who remain alive, the avoidance of the greatest loss of all will be well worth it, and the human capacity for adaptation and growth in the face of new circumstances is and has always been remarkable. Furthermore, the continued presence of individuals from older generations would render this transition far more humane than any other throughout history. After all, entire generations would no longer be swept away by the ravages of time. They could persist and preserve their knowledge and experience as anchors during times of change.
Every day, approximately 150,000 people die, and approximately 100,000 of them die from causes related to senescence. If those deaths can be averted and the advent of indefinite life extension accelerated by even a few days, hundreds of thousands of irreplaceable individual universes would be preserved. This is worth paying even substantial costs in my view, but, fortunately, I think the other - economic and societal - effects that accompany life extension would be overwhelmingly positive as well.