It's nothing (well certainly not everything) to do with people's assumptions. There's a ton of research around how people simply stop paying attention when there's no reason for them to pay attention 99% of the time. It doesn't even need to be about them pulling out a book or watching a movie. It can simply be zoning out.

Maybe, as you say, it's feasible today or soon to better handle brief distractions but once you allow that it's probably dangerous to assume that people won't stretch out those distractions.