The cautious majority believes that human life span will continue to increase, but only incrementally, much as it has done for the past few decades - both life expectancy at birth and life expectancy after 60, due to the continued introduction of new medical technologies. (Which proceeds far more slowly than it could, thanks to the heavy hand of the state). One faction of epidemiologists even argues for the possibility of a dip in overall life expectancy as present trends in obesity take their toll - to their eyes the consequences of being overweight look set to outweigh modest gains due to advances in medicine.
To my mind, arguing for incrementalism in any trend relating to medicine at the present time is choosing to go against the tide. The biotechnologies that underpin advances in medicine are going through a period of massive, revolutionary change. While it is true that organizations such as the FDA do pretty much everything short of shooting scientists to slow down and increase the cost of turning research into therapies, the rapid pace of progress in the life sciences will win through.
Allow me to put forward a historical analogy: standing in 2012 and arguing a case for gentle future changes in life expectancy over the next few decades, based on the past few decades, is something like standing in 1885 or so and arguing that speed and convenience of passenger travel will steadily and gently increase in the decades ahead. The gentleman prognosticator of the mid-1880s could look back at steady progress in the operating speed of railways and similar improvement in steamships throughout the 19th century. He would be aware of the prototyping of various forms of engine that promised to allow carriages to reliably proceed at the pace of trains, and the first frail airships that could manage a fair pace in flight - though by no means the equal of speed by rail.
Like our present era, however, the end of the 19th century was a time of very rapid progress and invention in comparison to the past. In such ages trends are broken and exceeded. Thus within twenty years of the first crudely powered and fragile airships, heavier than air flight launched in earnest: a revolutionary change in travel brought on by the blossoming of a completely new branch of applied technology. By the late 1920s, the aircraft of the first airlines consistently flew four to five times as fast as the operating speed of trains in 1880, and new lines of travel could be set up for a fraction of the cost of a railway. Little in the way of incrementalism there: instead a great and sweeping improvement accomplished across a few decades and through the introduction of a completely new approach to the problem.
This is one of many historical examples of discontinuities in gentle trends brought about by fundamentally new technologies. Returning to the medicine of the present day, there are any number of lines of work we could point to as analogous to the embryonic component technologies of an aircraft in 1885. They are still in the lab, or only being trialed, or still under development - but they exist in great numbers. There are the SENS technologies; a range of advanced applications of immunotherapy; targeting methodologies to safely destroy specific cell types; organ engineering; and others. Just because we can't see the exact shape of the emerging technologies that will be constructed atop these foundations doesn't make them any less likely to be created: great changes are coming down the line in medicine. The future is not one of steady and incremental progress.