Need? Probably not. I bet it helps though (false positives, etc.)

>would be cheaper, faster detection, more reliable, easier to maintain, and more.

Cheaper than the phone I already own? Easier to maintain than the phone that I don't need to do maintenance on?

From a fun hacking perspective, a different sensor & device is cool. But I don't think it's any of the things you mentioned for the majority of people.

The same arguments were said for blind people and the multitude of one-off devices that smartphones replaced, OCR to TTS, color detection, object detection in photos/camera feeds, detecting what denomination US bills are, analyzing what's on screen semantically vs what was provided as accessible text (if any was at all), etc. Sure, services for the blind would come by and help arrange outfits for people, and audiobook narrators or braille translator services existed, and standalone devices to detect money denominations were sold, but a phone can just do all of that now for much cheaper.

All of these accessibility AI/ML features run on-device, so the knee-jerk anti-AI crowd's chief complaints are mostly baseless anyways. And for the blind and the deaf, carrying all the potential extra devices with you everywhere is burdensome. The smartphone is a minimal and common social and physical burden.

I've worked on some audio/video alert systems. Basic threshold detectors produce a lot of false positives. It's common for parents to put white noise machines in the room to help the baby sleep. When you have a noise generating machine in the same room, you need more sophisticated detection.

False positives are the fastest way to frustrate users.