> Most people dramatically under-size their air purifiers, or run them on a very low fan setting, and then they throw up their hands and just say that air purifiers don't work.

I believe something is better than nothing here. One of the biggest complaints against air filters is noise so maybe a good compromise would be to run them at full speed and full noise for a certain amount of time or something when nobody is in the room?

A false sense of security can be worse than nothing, because it prevents you from seeking out actually effective options.

I too would like such a "shy" air purifier, but manufacturers always seem to go the other way: when occupancy is detected they increase the fan speed.

Best option IMO is just to get an air purifier with a good CADR-to-decibel ratio and then (again) size it correctly. A surprisingly good option is something called the Airfanta 3 Pro, which is basically like those wildfire filter boxes except it uses PC fans.

bigger filter = less noise to move more air

The way to get air purifiers to really work well is to install them at the air intake, i.e. in the windows, or where the central air intake is, so all incoming air passes through them. I use indoor air purifiers too, but not as a substitute for ones at the intake. Note that tires and diesel fumes are prominent neighborhood sources of harmful particulates.

It is not expensive to run intake fans in the spring and fall seasons when active heating or cooling are not required.