> Why wouldn't that be useful? People be surprised how poor their air quality generally are inside

First because it’s the wrong solution to the problem. As I understand it bad air quality in a home is created by specific activities like cooking, vacuuming, or lighting candles/incense/smoking. So you solve the problem by turning on the fan when you cook, opening the windows when you vacuum, and by not using incense or smoking.

Second because you can have a dumb sensor and switch the fan on with your hand when it goes turns orange or red.

> I have about 70-80 "smart things" by now, but every single one is genuinely useful

We have different definitions of genuinely useful. I’m glad you find you setups useful tho - you do you!