Meaningless statistic increases

Someday perhaps someone will explain to me why anyone should pay attention to life expectancy numbers based on synthetic cohort assumptions. Under the synthetic cohort assumption, it is assumed that the mortality behavior of people born today can be well approximated by the mortality behavior of people who are old today. This is ludicrous. It ignores all technical change in health care as well as the major behavioral changes that we know exist across cohorts due to, for example, changes in BMI and in smoking. How can these numbers do other than mislead?