If you have a subject moving across the field of view, (a star, to take your example) it will describe an arc across the sensor. For any given shutter speed, that arc will be the same whether you are using a 20MP sensor or a 50MP sensor. So if you use a faster shutter speed on the low MP sensor, won't that also reduce the star trail in the same way?
The arc across the sensors would be the same when measured in millimeters, but not in pixels. In the example I'm comparing star trails of 11 pixels which can be scaled down to 1 pixel. If I want an image with no star trails (a 1 pixel star remains a 1 pixel star), the high MP sensor requires a faster shutter speed to not produce a trail. In my case I need a 2X faster shutter speed on the M6II compared to the 1DXII if I want no star trail using the same lens.