You're right that they're expensive and need a decent activation force. They also are quite large and make a quite loud clicking noise which might be annoying for a sensor application.
Sure you have to press it very slightly harder than a regular switch and the travel is a little more, but not by much.
I think the doorbell cost like £20 in 2020, actually lemme check Amazon...ah in fact it was 15.99£ (now it's £20.99 hmmm) "TECKNET self powered doorbell".
If any of those doors are important for security, then I'd want something an intruder can't easily jam or spoof.
They're good but relatively expensive and relatively large. So I can kind of see why this might make sense. On the other hand having to put ultrasonic microphones all through your house is clearly much worse than a radio receiver, so I'd say these are a bit of a gimmick still.
Indeed. At best, they're an "emitter", a "proxy", a "relay", a "transformer", or some combination thereof with "sense" or "marshall" that indicates the transformation of input to output a la "sense proxy emitter" or "marshalling sensor".
There's probably no reason why these kinetic switches can't also be used for detecting other events like doors opening/closing etc. I feel like a radio signal is a bit more reliable and easier to detect than high frequency sound.
I also think calling these a "sensor" is a bit of a stretch. They detect events but have no knowledge of the current state of the thing they're sensing. E.g. the can detect a door opening/closing, but have no idea if the door is open or closed at a given time