I didn't say that lifespans would necessarily become greater in that period, but several decades is time for the rates to change quite a lot. And while public health has become worse in recent decades in a number of ways (obesity epidemic, lower rates of exercise,) a technologies have been developed which improve the prognoses for a lot of ailments (we may not have cured cancer yet, but many forms are much more treatable than they used to be.)

If all the supposed medical discoveries I hear about on a regular basis were all they're cracked up to be, we would already have a generalized cure for cancer by now and already have ageless mice if not ageless humans, but even if we assume no 'magic bullet' innovations in the meantime, the benefits of incrementally advancing technology are likely to outpace decreases in health if only because the population can probably only get so much fatter and more out of shape than it already is before we reach a point where increased proliferation of superstimulus foods and sedentary activities don't make any difference.