This might have been true in the age of modems and two hours of internet access per day, but not any more. Most people have exactly one device directly connect

This might have been true in the age of modems and two hours of internet access per day, but not any more. Most people have exactly one device directly connected to the internet (their router), and that router has hours of downtime per year at most. Most of that downtime (if there is any) is due to electricity outages, and so correlated with downtime of other consumers. When there are no outages, very few consumers can be left without an IP address. I'd expect the savings in IP space to be <1%, although I have no hard data here and might be missing something. This doesn't apply to cellular carriers, there are plenty of subscribers who maintain their plans but disconnect often. This includes people living abroad (who normally use another country's SIM), or even people who turn off their work phones on weekends.