People typically use a mouse with 2 digits (pointer finger and middle finger). Motion is captured in 2 dimensions. Toss in a scroll wheel an extra thumb button

People typically use a mouse with 2 digits (pointer finger and middle finger). Motion is captured in 2 dimensions. Toss in a scroll wheel an extra thumb button and that’s about 6 different inputs. 2 mice could theoretically be 12 different inputs. The inputs aren’t evenly distributed though, in terms of ease of access. The function of the left click button isn’t swappable with the x-axis of the trackball, for example. —- Now compare that with the keyboard. A keyboard with just 3 rows of 10 keys each is 30 different inputs already. Each input is fairly easy to swap - any finger can (relatively) easily hit any key, regardless of function. The only exception is the x-y motion of the trackball. —- So it seems logical that the best option would be the combination of a mouse and a huge number of keys.