To start, lifespan has already increased dramatically since prehistory, and particularly so in recent modern times. Some may argue that developments in medicine and sanitation are the key source of this increase, but I would say it's a positive feedback loop. As these developments allowed us to live longer, people had more time to put their minds to creating more beneficial technologies, which in turn allowed for a longer lifespan, etc. And the benefits have trickled down, I may add, at a faster and faster rate. Although the rich do benefit by affording themselves the very best there is to offer, many of us still have access to wonderful technologies. Within a few short years what was once only available to the rich soon becomes a staple of the average consumer. A greater increase in lifespan could be an enhancement on this process.
Looking towards the future, a society that has already achieved life extending technologies will have some key beneficial changes. For one, with an extended lifespan people would feel less inclined to have children. Although many still most probably would, they may not consider it until they are very old in our current terms, perhaps only a couple per century. Such a slow in birth rates would lead to a much more manageable human population growth and allow responses to things like overpopulation to be dealt with well in advance. The "stretching" as it could be said, of a person accomplishing typical life goals (having children, buying a house, starting a career, etc.) could be beneficial to the education system as well. If a person has exponentially more time to spend learning, and trying new things, who knows what kind of innovations could follow, along with a more educated society in general. Just as an extended childhood has been advantageous for humans relative to other animals, an extended "childhood" relative to our current notions could hold unseen benefits.
A society with a longer lifespan is also more likely to take proactive measures against future threats. Our current society is more reactive because many of the threats we are dealing with will not much affect our lives, or at least not for many years to come. If you knew for example, that the worst possible effects of climate change would have a direct impact on your life though, you would be much more willing to take action now. Although at it's base it seems selfish, this very behaviour could save humanity as a whole by allowing us the opportunity to think ahead and actually take measures in advance of serious catastrophes.
It's always good to weigh the pros and cons of a new technology, and the promise and peril it brings with it. Our ability though to deal with the consequences of many technologies is enhanced by lifespan increases, not detracted from.
