> A fan that switches on when air quality falls below a threshold? Not that useful in a living room,

Why wouldn't that be useful? People be surprised how poor their air quality generally are inside, unless they already measure it, making it better sounds useful in oh so many ways.

> i know, because of the 15-20 smart things i have only one or two are genuinely useful.

What are those things? I have about 70-80 "smart things" by now, but every single one is genuinely useful, otherwise I wouldn't install them in the first place. Lots of open/closed sensors, soil moisture, temperature+pm2.5 sensors, water taps and so on.