the notion that teenagers in the 2040s and 2050s would all be familiar with Harry Potter or Beyonce struck me as unlikely

Normally I would say it's not unlikely for teenagers to be familiar with kids' books of decades past, since adults often recommend books that were formative for them. (As an '80s kid, I read many books from the '50s and '60s.) But in the case of Harry Potter specifically, the author's notoriety makes that seem questionable, unless we're talking about a dystopia where her attitudes have become the norm.