Is the worldwide race to keep expanding mobile bandwidth a fool’s errand? Could maximum data speeds—on mobile devices, at home, at work—be approaching “fast enough” for most people for most purposes? These heretical questions are worth asking, because industry bandwidth tracking data has lately been revealing something surprising: Terrestrial and mobile-data growth is slowing down. In fact, absent a dramatic change in consumer tech and broadband usage patterns, data-rate demand appears set to to...