Dementia is most certainly not a runaway train given that incidence rates have actually been declining for a couple of decades. 

While the prevalence of dementia is expected to increase because of an aging population, downward trends of incidence have been observed in a variety of studies within different countries and populations.

https://www.nytimes.com/2016/11/21/health/dementia-rates-united-states.html

http://www.mdedge.com/neurologyreviews/article/77918/alzheimers-cognition/dementia-incidence-declining