From Zack Lynch's Brain Waves, a good example of the way in which conservative forecasts based on simple extrapolation of trends fail to account for the reality that is the future:
Twenty percent of us, according to a Rand Corporation study, are going to get cancer or another rapidly debilitating condition and we'll be dead within a year of getting the disease. Another 20 percent of us are going to suffer from some cardiac or respiratory failure. We'll suffer years of worsening symptoms, a few life-threatening episodes, and then eventually die.
But 40 percent of us will suffer from some form of dementia (most frequently Alzheimer's disease or a disabling stroke). Our gradual, unrelenting path toward death will take 8 or 10 or even 20 years, during which we will cease to become the person we were. We will linger on, in some new state, depending on the care of others.
To the end of this, we should mentally add "provided that nothing in medicine and research changes greatly." You should add that endpoint to most reports on the future of health and longevity (with a few enthusiastic exceptions). Excepts like the above are not predictions; rather, they are warnings, just as any serious consideration of your own personal future in the absence of advanced medicine should be a warning. This will be our fate unless we do something about it - and there is much that we can be doing! Folk like you and I can make a great deal of difference by advocating, supporting and encouraging medical research into cures for age-related disease and strategies to prevent the root causes of degenerative aging.
The future is what we make of it - and that is just as true for medical progress, health and longevity as for anything else.