the way Waymo does it with the spinning sensor

There's a reason they do that, he actually covers that in the video. Lidar spins a single line many, many many times a second. Processing the differences in that line scan to scene makes the point cloud generation many times easier allowing the scan to be exponentially more dense.

The iPhone uses a diffraction grating to shoot static dots at your face and looks for the subtle movements of your face and phone to generate a 3D scan.

The diffraction method is tiny and good for static identification but bad for high-speed outdoors.

The spinny towers give it a better field of view. you could probably put shorter towers on each corner, or even build them into the body panels, but it's a delicate, expensive instrument and it's not what's currently holding back self driving anyway :)

Everyone and their dog uses radar for distance sensing for the adaptive cruise control. You take the same migh speed sensor and use it for wall detection. It's how the emergency stop functions work where it detects a car in front of you slamming on the brakes.