The timing can seem paradoxical — the star power of Julia Roberts and Andrew Garfield deployed to examine a world stripped of so much glamour and currency. The past several years have witnessed a steady churn of journalistic lament over the fate of the humanities on American campuses. The prognosis, at least if Ian McEwan is taking measure, shows no sign of turnaround. His new futuristic novel, “What We Can Know,” is lodged in a 22nd century in which there seem to be just two remaining historians on earth. According to federal statistics, the percentage of college students majoring in English or foreign languages and literature plummeted by a third during the 10-year period ending in 2022.