I must begin by nitpicking on the title: Billions must die? Which billions? ACKSHUALLY, population growth around much of the developed world has been on the decline. It's the comparatively less developed countries producing the most young people.
Nonetheless, smartphones are getting cheaper and more accessible so they're hit with the brainrot anyways—so much that older generations aren't safe, either. So, yeah, billions.
As one guy puts it, concerning.