They'd better work on "normal" sized pixels with extra performance rather than smaller pixels that are barely as good as the previous gen. Alas, big numbers sell
They improve sharpness if optics can resolve them. Also, having more data will might ease autofocus algorithms and computational photography techniques.
Low light and DR depend solely on the quantity of light falling on the sensor. That depends on scene illumination, sensor size and lens aperture,not on pixel size.
However, having a huge resolution is a waste of storage and processing power if the optics can resolve much of those pixels. And it could slow things down.
To my understanding - If the pixels are smaller, the gaps between pixels are likely to make up a larger proportion of the sensor, so you do lose some light from this process, although there are definitely upsides to extra pixels if there is more detail to be resolved