Intelligence Squared[1] did a debate on this topic. It's definitely worth listening to.

To summarize a couple of the more compelling arguments in the camp _against_ extending lifespans:

1. Turnover of ideas from the older generation to a new generation with fresh ideas. This turnover is healthy and benefits our society's ability to progress.

2. Negative impact on our decision making processes. We construct a narrative of our lives and often the decisions we make are based on how they impact our personal narrative, knowing there is an 'end' to our story makes us value each decision we make, and thus make better decisions.

[1] http://intelligencesquaredus.org/debates/past-debates/item/1... (warning: video will autoplay)